Before answering this question I want to point out that the clip you’ve identified doesn’t quite square with “tropical climates”. In the clip mentioned, the guy draws two lines and says that people within these latitudes tend to be bad fighters, though at this point he gives no reasons. However, the two lines seem to be to the north of what has traditionally been described as “tropical”, which is between the Tropic of Cancer and the Tropic of Capricorn with a particular climate. Egypt, for example, is generally not a tropical country. The only part of Egypt that is tropical is, ironically, the southern sliver of the southern half of Egypt, the same southern half of Egypt that was responsible for unifying Egypt in the first place.

If you asked me to guess, based on this clip alone, I would say he used his rudimentary knowledge of history to make a false assumption about which states had lost most wars, then drew lines across them based on what he thought to be the northern borders of these states, and then falsely claimed that they have all “lost the vast majority of military encounters with opponents to the north of them”, and perhaps later tried to justify it through some badly outdated theories that he has misunderstood anyway. I daresay if you look at the three cultures he mentions (India, Egypt, southern China), you would find that if they have lost wars, there are far more factors at play than “bad fighters” or “climate”. One example can be found here

Earlier in the video, he cherry picks Egypt as an example and says that from 500 BC to 1950 CE, it was never under the control of the local population. However, the list of foreign ruling dynasties he mentions includes the Persians, the Arabs, the Berbers and the Kurds, all of whom fall within his latitudes. How can they possibly have kicked out the armies of the great Roman and Greek civilisations to the north if, as he claims, they were bad fighters?

I also think he realised that there were plenty of exceptions to his sweeping generalisation, and thus he put in the disclaimer “don’t count desert and mountain tribes”, conveniently preempting the very obvious example of the Umayyad Dynasty conquering the Iberian Peninsula. This is a very odd disclaimer, because earlier in the video he makes a big deal of irrigation societies being autocratic and having disaffected populations that don’t fight very well, but then how does he suppose desert civilisations got their water if not from irrigation? Or does he think it was precisely the lack of irrigation that turned the Umayyads into Fremen-like warriors?

Also, notice how the southern line he has drawn very conveniently stops at the southern tip of present day India, beyond which there is just ocean, thereby sparing him from justifying why India didn’t also face invasion from the south.

So anyway, what I’ll do is first point out what the world would look like if, indeed, there was a part of the world that was full of autocratic irrigation societies and riddled with disease. Then, I’ll look at some of the problems with his claims, focusing on Southeast Asia, which is one big tropical zone, and show some examples that disprove his claims.

First, since the guy is all about alternate history, let’s just imagine for a minute that there really does exist a belt which is full of disease and parasites. By his logic, constantly living in such an environment, inhabitants of such a belt would have evolved immunities, or resistance, or at the very least figured out cures to these diseases and parasites.

He himself says at 4:14, when talking about disease-ridden Florida,

After a couple of generations they [white settlers] were able to gain immunity.

And at 4:32 that

Interestingly enough, black Americans have a genetic mutation that lets them deal with malaria, which is a big reason they were brought over as slaves in the first place.

Such a belt would be impervious to attack by anyone who lived outside the belt, because anyone setting foot in such a deadly environment without immunities would have immediately been killed, or left in a weakened state, making them easy prey for the locals.

Let us further imagine that this belt were full of autocratic irrigation societies that needed to organise in order to build irrigation systems. Such societies would be highly autocratic, run by powerful leaders who had to be obeyed, so that they could achieve complex irrigation projects.

So basically, you’d have a belt filled with groups of warriors excellent at following orders, led by powerful leaders, possessing advanced immune systems and healing technologies, capable of advanced feats of engineering and living in a homeland completely impervious to attack. Far from being crappy warriors, such a group would probably beat the living daylights out of the rest of the world without breaking a sweat. In fact, all they would have to do is blow their noses on blankets and then catapult them into cities to spread whatever deadly disease the temperate natives had no exposure to.

I want to be fair and say that he mentions that temperate diseases would kill lots of southerners if ever they tried to move north, thus giving rise to separate northern states. However, if this were true, we would never see any form of migration from tropical to temperate zones and vice versa before disease wiped out settlers and soldiers in horrific numbers. By this logic, Julius Caesar would have dropped dead before reaching the British Isles. And even if it were true, it completely negates his point about civilisations within his latitudes being comparatively weaker because of disease and parasites.

(Continued in reply)