How the hell could they lose against humans 1500 years ago...

The 'Americans' couldn't even handle Baghdad. Many powerful empires have been defeated. The British Empire, Napoleon's Army, the Empire in Star Wars.

The Espheni probably sent a small group to Earth 1500 years ago and underestimated the will of the natives. They probably thought the natives would worship them but they picked the wrong tribe to mess with.

Well first of all, there were native americans in america. Secondly, you assume that the "battle front" was in america. Third, most people back then would have been less "pampered' than today given how life typically was a lot rougher for them back then. Forth, the Espheni would not have been able to benefit from a lot of things(resources) back then as they would in modern day. For example, there would have been a far smaller supply of scrap metal to aid them in the construction of ...well, anything; there would have also been no petroleum to help power their machines.
And of course, they probably just totally underestimated humans back then.

It is a simple answer to what should've been a simple question: the humans back then were REALLY HUNGRY1

You have to suspend common sense. Then, you'll get it.

The ability to travel through outer space alone would amount to a decisive military advantage. A spaced faring alien race could hammer humanity into submission form orbit using conventional bombs as well as chemical, biological, and space rocks to cause tsunamis.

But, this is not the first time hollywood has done this. Try Star Wars: Return of the Jedi. Hollywood liberals apparently like this scenario of primitives defeating the technologic.

My take on all this:

first you must remember that the espheni are still testing with genetics ("skitterized humans)

... at some point I believe they say the Dornians were extinct not so long ago, they were enslaved and becoming the skitters...
.
Also the queen said she would come back when they were much stronger...
.

So, 1500 years ago, they had no skitters, probably few to no mech at all...
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A small number of overlords and the queen's daughter comes to earth and are promptly killed... (as seen, overlords can be killed rather easily with bare hands --Weaver--)

.

Still hard to swallow, but makes some sense...

A world without Rambo?? I cannot fathom.

It would've been easier to believe that her daughter had come to Earth on a Peace Mission, disarmed as a sign of goodwill and then the primitive humans killed her due to this.

Who is to say they even went to America the last time they came to earth and if we were to believe in the Dreamtime or the History of Ancient Egypt and the pharaohs they could have easily been defeated.

There were gods back then that had human bodies and animals heads with unlimited powers.

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