Dtyjlui

They have several disadvantages.

1. Attrition. Larger animals take longer to breed, it takes time and calories to grow. That means humans can replace their population faster. The number of babies is immaterial, most offspring die of disease or famine anyway. It took us several thousand more years to actually produce our actual reproductive potential, you need medicine and industrial farming first, your giants may have a higher potential but they won't be able to exploit it if anything they will be even less able to exploit it. What matters is maturation rate, especially with their higher caloric demand. And there is no way around them taking longer to reach breeding age. Over time humans will dominate more and more (consider they already start at a distinct advantage). Your giants also need to be eating a huge amount to keep up those abilities, nothing in biology is free, everything has a cost, stronger muscles means more they need more calories. Stronger bones require more minerals and slower growth. Your giants have completely wiped out any efficiency advantage size might give them, so humans have an even larger numbers advantage. 1 unit of crops are producing drastically more humans than giants. This gets even worse when you realize all the human spears are right at crotch height for the giants.




2. No livestock. This is a big one. Being big and strong does do much good when a horse bigger, stronger, and easier to replace. Humans have cavalry. Worse without livestock they food is less portable, humans can sustain fighting for much longer just because they will have better supply lines. One major weakness the Aztec had was corn has a comparatively short shelf life, they could not sustain war year round. Even better humans can use livestock for labor freeing up even more of their numbers for combat or more likely to build defenses, walled cities were common for a reason.

Sure their sense of smell might be good but it will never be as good as a dog's, so the humans have that advantage. Plus war dogs can bolster the humans' armies even further.

3. They are a warrior band culture, which means they suck at fighting actual wars. Tribal warfare is small scale, brief, and often for show. But the war your giants are walking into is a game of logistics, who can produce the most food, who has the best craftsmen, who is the best organized, who can sustain soldiers for longer. Your giants make for good raiders and poor soldiers. They are not fighting tribal bands of humans, in 400AD they are fighting empires. Humans have incendiary devices, ballista and stone walls. Humans can sustain war for years without stopping. War bands mean small numbers, tribal bands can't collect in large armies for long, they start to starve, humans, on the other hand, can field armies of hundreds of thousands.




4. Smell, A better sense of smell may actually work against them, humans will be able to use rotting livestock as both psychological and biological warfare.




5. Size is not always an advantage. Wait until they get to forests or jungles where size becomes a hindrance. Humans can also better utilize water, humans boats will have shorter drafts meaning they can travel shallower waters, meaning humans forces will be more maneuverable on the large scale. One of the biggest advantages Vikings had was they could show up anywhere at any time, they could sail even the shallowest of waters. Snow will be even worse, giants will be big and heavy and even strong snow will not support their weight. Not to mention armies of giants will have trouble feeding themselves in cold climates.




6. Inflexibility, double muscles animals are very inflexible, those muscles get in the way of each other. Giants may be faster on the straightaway but humans will be able to run circles around them up close. Giant will get surrounded easily, and will not be flexible which means they will worse even worse at climbing and traveling through forests and jungles.




7. endurance. Double-muscled animals have far less muscular fat, meaning they have drastically reduced endurance. So even in an individual battle humans will be able to fight longer than giants, they can simply wear them out.

Not enough females.

They can't replace attrition losses. The humans have figured this out and are going for the balls, which are, incidentally, at a very convenient height.

Once sufficient males are gelded, the remainder are too busy trying to keep their numbers up to worry about conquest. The gelded males tend to lose interest in further conflict in the immediate aftermath, while shortens their life expectancy severely. Those that survive are driven from their clans by the mocking from their peers (the perils of a jock based society) and end up as mercenaries or pets of human overlords.

Bigger giants!

It was not food pressure or warfare that drove these giants north. Like the Goths being driven before the Huns, your giants are chased out of their ancestral home by larger giants!

These 20 foot tall giants originate to the south, in Antarctica. In the first wave, 8 megagiants trickle into New Zealand. In the far larger second wave 5 million of these monsters move in to Oceania, displacing the entirety of the minigiants north into South Asia and China.

The Antarctic giants are physically superior to the minigiants in every single way.

• 20 feet tall average


• Four armed (some of them) and four breasts (most of them).


• 50% higher vertical leap than mini giants.


• Flexible bones and thick layers of fat, allowing enormous damage resistance.


• Born knowing the art of savate.

This is all true just for the females. The males look like ordinary sized dudes but everyone is cool with that.

Plus they are culturally superior - these giants have a bonobo like culture, held together by bonds of grooming, consensual sex, back rubs, and long extemporized songs about the beauty of nature and how awesome other individuals in their culture are. The one exception to their gentleness is in regards to minigiants, whom they rip to shreds on sight and then eat the shreds raw. Sometimes they challenge them to feats of strength first, and when the minigiants lose (always) they are then ripped to shreds and eaten raw, or with a little pepper.

These Antarctic giants (ok, giantesses, with regular dudes along) keep coming, relentlessly extirpating the minigiants everywhere except for their mountain sausage party hideways where they pout and grouse and continue their manly wrestling warrior cultural ways.

Fortunately for regular humans, the big sexy giants dig what we do! Regular sized humans are considered entertaining and attractive, especially if they can sing. Megagiants quickly assimilate into the human population. Megagiantism is a recessive trait and by the year 1000, the only trace of this wave of polar saviors is the occasional birth of a girl who grows up to be remarkably tall and has 4 arms.

First think about the message of your story; what you choose should depend on what you want to say to your audience. I can think of 6 categories. For historical realism and story depth, I'd suggest picking several causes, from more than one category.

An inherent human strength: this says, "humans are magically wonderful"

• mental (tech, smarts, wisdom, risk-taking, adaptability)


• physical (speed, agility, stamina)


• emotional/social (cooperation, leadership)

An inherent giant weakness: this says, "at least you don't suck"

• mental (stupidity, ADHD, OCD, stubbornness, laziness, migraines, insanity)


• physical (food/oxygen/heat/etc needs, poor longevity, poor vision, can't jump, kryptonite, moonlight makes them were-ducks, etc)


• emotional/social (low stress tolerance, poor communication, cultural problems, leaders' mistakes)

A greater power appears: this says, "everyone is powerless"

• disaster (I vote meteor strike)


• disease (no... please, don't)


• another race or creature(s)

Humans gain a strength or overcome a weakness: this says, "you have magical potential"

• they nuke 'em

Giants gain a strength or overcome a weakness: this says, "nope, it's just you that sucks"

• they become disillusioned with war


• they become more interested in some other venture


• they develop kindness

A choice: this says, "it's up to you"

• in a situation of matched wit, one human makes a wise decision or sacrifice

So these giants arrive in Eurasia in droves, driven by hunger. They land in coastal settlements taking everyone by surprise. But as they move inland news of their arrival quickly spreads and armies are readied to meet them.

Sheer numbers will get them quite far, but likely at heavy losses. This will be for several reasons.

Horses

A lack of horses provide a massive disadvantage. Organising groups of giants without mounted messengers is going to be a lot slower than organising humans who do have mounted messengers.

Horses also allow humans to move stuff around a lot easier. Food armies, weapons, armour, siege engines, etc.

Horses use in actual fighting will also be a factor. Mounted archers can close any advantage giants have with range quickly and decent heavy cavalry will make short work of combatants with iron age weapons.

Tactics

Judging by where these giants have come from, it's doubtful they have had much experience of siege warfare or warfare in large open areas.

These giants need to eat a lot, so pin them down anywhere and they will quickly starve. Plus, because they are so big, they are more likely to be susceptible to siege weaponry. Their size is going to make it difficult to occupy existing defensive strongholds and it takes a lot of time to build suitable stone defenses. Wooden structures aren't going to be much use against trebuchets and ballista.

On open ground they are going to meet a tactic they probably have never seen, a shield wall. A well formed shield wall of sufficient depth can stand a charge from heavy cavalry, so these 9 foot giants aren't going to be much of a problem, especially if they don't attack/defend in a line with suitable shields.

As humans withdraw from invading giants they take all livestock with them and scorch the earth behind them leaving no food for these giants who must need a lot to eat.

Weaponry

Steel is going to be a major advantage for humans. From spears, arrows and swords to armour and embossed shields, the humans will have the advantage in the weaponry department.

Infighting

If they are leaving their homelands because of internal strife, they will not all be from the same faction and will be as likely to fight each other as they will be to fight humans.

Cannibalism (maybe)

If these giants have been struggling for food for a while, cannibalism may be acceptable to them. So any time they eat one of their own, there is one less to fight.

On the flip side, this makes it much more likely that humans will be a viable food source for them.

Outcome

These giants won't last long anywhere humans can get a decent sized army together, but on difficult terrain, they are likely to have an advantage. This is where I foresee them settling, but food needs will make it difficult to sustain large numbers in these areas.

It won't take long though for an ambitious Warlord/King/Emperor to decide to train and arm these giants so they can fight their enemies in exchange for food. Give it 50 or so years and these giants will have overcome many of the disadvantages listed above.

Disease

This really did for the Native Americans; the Europeans had brought a bunch of the plagues and diseases from the old world with them. They were fairly resistant, due to repeated exposure. Whereas the peoples of the New World had never been exposed to the diseases at all, so they did not cope well at all.

Infighting

The giants value martial prowess. They aren't likely to back down from a fight with other giants. If one group of giants starts messing with another, the likely result is a war - instead of one group deciding 'screw this', in which case they might pick up and move.

Thus, after the initial displacement, the giants may be willing to sit on their new lands, or expand only slowly. Giant culture may well avoid large numbers of giants getting displaced into human lands by conflict with each other. They could view this manner of retreat as a coward's way out: sure, it's easy. Too easy, and it means you run when you should stand and fight.

Likewise, they might be more inclined to look to expand their territory into other giants' territory, rather than human territory. Why attack puny little scrawny things, that are so easy to kill you can't even really call it a fight, when you could be fighting a fight worth having against some giants who are worth respecting?

Food constraints

These giants will need quite a bit more food than humans. There will be plenty of places with marginal food production that can support human populations without much trouble, that would support a much smaller giant population.

When they've landed:

I would like to add another potential advantage which wasn't much considered so far: fortifications and machinery.

I would suspect that given their warrior nature, strength and size, the giants are not that used to large fortifications in war. They would need to invest a lot more resources and be better architects to make them work for their size, after all. Either way, human fortifications will make it difficult for the giants, simply because they are not built for their size.

Imagine a human attacking a dwarven stronghold - you can't even stand up straight!

Even if the giants have access to siege machinery, the humans can fire back, and the constraints on siege machinery are not really due to the size of their builders, but technical considerations, so both would be on even footing. Having steel available will very likely help in that regard, though, as does experience, so I would give humans a clear edge here.

Combine this with a scorched Earth policy, moving as many resources as possible into the castles and fortresses, and you may end up having a decent chance. Castles walls were quite sturdy, even if the giants are strong and tall, that doesn't help taking them down - they still need engines, or they have to storm the castle, exposing them to shot or more brute things like burning oil or scalding water ( more skin to burn ).

These defenses were, by the way, already established generally speaking, so would need expanding more than building in most places.

Since the humans have steel, consider also machines like the Roman scorpio or ballistae, which were powerful. They could crush human spine at a distance, I doubt a giant would fare well being hit by one of those!

Very importantly, though:

I would immediately go on the counter offensive where it matters most: the giant's supply lines. Given they are supposedly on a similar, perhaps lower tech level, the Ships they use cannot really be that much better than human ships. But they must carry Giants, and a much larger amount of supplies. Build warships and attack them at sea. This will dry out their ( pretty impressive, perhaps unrealistic ) numbers of reinforcements, and make the whole invasion more manageable.

Which reminds me of another (East) Roman / Byzantine invention: Greek Fire. Giant ships burn just as well, but I would suspect given their size, they cannot build as many as easily: wood will quickly get sparse.
While giants would perhaps have an advantage boarding ships ( maybe not, with their size, they may be outmanouvered ), this does not count for much in naval warfare if you can attack the Ship itself.

I think this can be as simple as guerilla warfare, Vietnamese style. Humans dig networks of tunnels and hideouts. Too small for the giants to enter.

Humans try to lead the giants into dense forests or jungle where the humans can take advantage of being small. Inside the dense vegetation the humans can leverage their knowledge of iron by using machetes to cut away only the low vegetation. This will cause the giants not to have a good overview of the battle grounds. Booby traps will also be efficient in this scenario.

Perhaps you should read "The Mad Moon" by Stanley G. Weinbaum where the protagonist gets in trouble with six inch tall ratlike intelligent beings called slinkers by human and spends much of the story desperately trying to escape from them.

http://gutenberg.net.au/ebooks06/0604211h.html1

Another factor that would help the humans in south Asia and Southeastern Asia in their fight against the invading giants would be the many giants in their armies even bigger than the invading giants.

Giants?

Yes, war elephants much bigger than the humanoid giants. There would be tens of thousands of war elephants at any one time in Asia during that time period and several times as many tame elephants used for civilian purposes.

War elephants were sometimes armored. War elephants sometimes had long blades attached to their tusks and were taught to use them in battle. War elephants sometimes held huge swords, or long iron chains, or other weapons, in their tusks and used them on enemies. Wild and domestic elephants sometimes throw rocks or tree trunks.

Elephants have carried giant crossbows, machine guns and small cannon on their backs, and could carry small trebuchets on their backs.

Circus elephants have been trained to do many unusual things, so if someone invents some sort of repeating crossbow or repeating trebuchet I can imagine elephants trained to turn a crack to constantly load and fire the weapon while humans aim it at giants.

Even cow and young elephants too small to fight the giants hand to trunk would be strong enough to power various long range weapons to shoot the giants with, if the humans are smart enough to invent them.

Starvation

The main reason why evolution doesn't favor much larger primates than us or gorillas is the core issue that lifestyle two individuals who each need 2000 calories/day can get more food than a single individual who needs 4000 calories per day, no matter if it's gathering, hunting, herding or farming. These giants are physically superior to H. sapiens in every way except the most important physical thing that counts which is calorie efficiency. There's a good reason why we're evolved to not grow extra muscle unless absolutely neccessary (exercise) and a large calorie surplus is available; it would be a trivial genetic change for humans to be much stronger, but there's evolutionary pressure away from this. For hominids being much stronger is generally an evolutionary drawback, not an advantage, because of the calorie cost of building and maintaining that muscle.

A 9ft tall, 7 times as strong, 50% as fast individual has more mass and a faster metabolism than humans do, they need five or ten times the food as humans do. They might be able to defeat a human population five or ten times their number, but if they exterminate them, then they can't farm as well as that population (a giant would be better at farming/herding than a single human, but worse 5 humans), especially without livestock, so they can't live off of the conquered land. At 435 AD, there's barely any food surplus already above what's needed to sustain the current human population, so they can't live off of tribute from conquered populations, not in numbers as large as this. Perhaps 1 giant could be fed from tribute for every 100 human farmers? But can they sustainably stay in power if humans have a 100-to-1 numerical advantage?

Let's suppose they land in Korea and exterminate the local human population. What next? The number of giants needed to do that can't possibly survive in Korea, their lifestyle can't sustain a population density nowhere close to that (for that matter, Polynesia and Australia can't possibly supply enough food to grow 500,000 or 50,000 new giants a year, not at this technology level). Which is the key point - given your description of the giants, they would always have the inferior armies, because if a giant is as good in combat as 10 men, then the same amount of farmland that's needed for that giant to make food for himself (taking into account that such a giant would be much worse in food production than 10 men, the "physically superior" combat aspects don't help him that much) would supply 20 or more men.

And what do they do when they reach the steppes of Central Asia? Without livestock, it's a food desert for them; they can only make expeditions through it with stored supplies unlike nomadic humans who can live there in large numbers and raid their supplies.

The same argument as with Neanderthals would apply. They were stronger, smarter and more technologically advanced than us, but homo sapiens were more calorie efficient. So in the long run, we outcompeted and assimilated them, not vice versa. The same would make sense for the giants.

If they don't intermarry so their offspring inherit immunities, then disease will decimate them and the survivors will be mopped up fairly easily. Whole villages and even Islands in the Pacific were depopulated after European contact due to disease. Others when disease on another Island disrupted essential trade supplies.

espionage and spies

Sneak a human in at night and kill war chief A son with war chief B's axe.

Now they kill each other.

Any rumors or information they can plant suggesting giant group 1 will attach group 2 with fabricated evidence as needed.

Kill a small group of them with overwhelming numbers from group 1 and leave the weapons/armor for group 2 to find. (or plant it on them) Then everyone else will think group 2 killed group 1 and not you. A 3rd party attack them for killing group 1. Its all about kill giants that will be missed, and trigger revenge attacks.

After a crushing defeat against the first fleet of giants, humans collectively retreat and give up control to the giants. The humans still exist, they are far larger in number after all and they just live as an oppressed people. A giant may eat all of one villages livestock, a group of giants may drive out the humans in a specific area to control the land, but humans as a race would still be around and plentiful, just suffering a lot.

I imagine this would go similarly to any colonization ever, except perhaps because of the giants overwhelming strength the humans may give up any resistance even faster. If there are small scale fights, massacres, last stands ect this still works fine since you are talking about such large numbers. Just because a colonizer can commit mass genocide on the native peoples does not mean they will.

"But the giants are a warrior race and it will be unnatural for the worldbuilding if they just decide not to fight the humans" - Well, I think by making the giants so much stronger than humans you have created a convenient out for this one. Just by establishing that the giants view humans as inferior (in terms of combat) a social idea that humans are not worthy opponents and there is no "honour" or whatever in defeating a human it makes sense that they would not constantly want to fight humans. Why should they? For an analogy, the Mongols were pretty famous for being combat-oriented. But they did not go around taking up arms and duelling every goat they came across because why would they? obviously goats are not a big enough issue for the Mongols to bother about.

Basically, humans don't have to be able to win fights to survive utter destruction. An unpeaceful and oppressed co-habitation is entirely plausible.

Well it is my understanding that for a giant race that is hell bent on killing humanity(I am assuming they are not be killing humans for sale of keep fit excercise routine , and also not because they are jeleous of our good genes and beautiful looks as against there ugly deformed ones ! No Sir... It is obvious it's because we as a food source as meet food are absolutely delicious... Do you see they might kill a good number of us every day but it won't be to finish us justtocook us and eat us.. the rest you can understand dears.

Cold

Humans could be forced into colder areas. These giants come from Australia so they are used to warm weather thus their immune system is weak against cold, not to mention how much heat moving their body is producing. They also have weak, light armor which doesn't protect them from cold. They would also leave areas that are cold in the winter and warm in the summer alone as there is no point in colonizing a land which is not habitable half of the time. This land could be used to allow giants to interact with humans through trading in summertime or it could be a war zone which humans have to protect so they can grow food for the winter.