I should think most fighters are not defending "humanity" in general, they are fighting on behalf of the people they know and the way of life they think can be salvaged.
Your emotions and values are tied up in other people; the residents of your village, home town, neighborhood. The people you see in your day to day life. When you go to war, those are the people you imagine saving, and those that were lost are the people you imagine exacting vengeance for. Along with the people that fight beside you, your fellow warriors.
If there is a predominant emotion in war, I think it is "resolute determination", portrayed pretty well in "Saving Private Ryan". Fear cannot be sustained for a long time, you may feel fear in a fire fight, but you return to the baseline state of "We're doing this, even if we die, to secure a future for those we love."
I love my family and my town, the babies, the kids, the insufferable high schoolers that know it all while knowing nearly nothing, the young women, the young mothers, the old mothers, the shopkeepers, the elderly bridge players telling each other the same endless stories for decades, I love them and that's why I fight. I love them more than my own life, and I will give it up if that is the only way to give them a chance against being slaughtered or enslaved.
IMO it isn't the excitement, or dreams of being a conquering hero, it is about preventing terror and horror (or any more terror and horror) from being inflicted upon the people you love, in particular those that you know have no chance of defending themselves if you do not succeed. Incidentally, that might save "humanity", a nice bonus, but that is not the main attraction.