I didn't say only people with strong religious convictions can resist conquest by stronger forces, I just said that it's hard to conquer people with strong religious convictions.

To say what you're saying responsibly you need to establish that their religious conviction is the reason why. Given that we see it elsewhere and across religions it seems much more likely to me that simply being invaded by a more powerful state is a strong enough source of this conviction.

Aside from that the "they're happy to die because their religion says they get virgins" is something I remember my second most racist uncle saying after sept 11. It's not even really true? It's a very narrow reading of a particular passage, that, yes, extremists have gotten a lot of mileage out of. But from my lay understanding of islam does not feature heavily in the personal religious experience of most muslims.

Again unless you have some particularly insight or expertise about the role of islam in motivating individuals in conflict I think you should reflect on where you got this idea and what you were trying to communicate by using it here.

The Qur'an explicitly states that martyrs will die in heaven, and most Muslims believe this.

"Those who leave their homes in the cause of Allah, and are then slain or die,- On them will Allah bestow verily a goodly Provision: Truly Allah is He Who bestows the best provision. Verily He will admit them to a place with which they shall be well pleased: for Allah is All-Knowing, Most Forbearing."

- Quran 22:58–59

The virgins part on the other hand is a lot more iffy. And Muslims generally believe it's not acceptable to kill a single innocent, since the Qur'an says so, so only a very small minority support suicide bombing like 9/11 (on the argument that since US citizens fund the military, they count as soldiers, not non-combatants).

Muslims themselves have claimed not fearing death to be a reason for past military success. For instance the quote "I have bought you men who love death as much as you love life" from one of the early Muslim generals https://en.m.wikipedia.org/wiki/Khalid_ibn_al-Walid . He said it to the Persians, whom the Muslim armies eventually defeated.

The issue is not whether these things are said, or even whether they are acted upon, but whether they are causally effective, and a number of war stories is insufficient to establish this.

An interesting point about Vietnam is that according to all participating parties for long stretches there was little to no support in the South for the Vietcong and everything was supported and orchestrated by the North.

https://archive.org/details/victoryinvietnam0000unse_u7j3 Victory in Vietnam : the official history of the people's army of Vietnam

Opened Soviet archives after the fall of the SU said the same.

The US basically always said that.