I never used nationalism b4 (a very veteran civ player btw and had used all others a lot) until I got invaded by the Zulu late game on emperor and they had kicked ass all game. I did some scouting with my planes THE INVASION ARMY WAS NEARLY 400 UNITS LARGE into my territory and my vassals (I used the military advisor to count). 300 modern armor, gunships and mech infantry alone (sry no screen shot). Anyway to make a long story short I switched to nationalism for the first time ever (and police state, not a first) over from free speech and democracy. WOW! The extra happiness and the drafting saved my ass. I only had roughly 60 modern armor upgraded from my panzers and 35 gunships. Not much else. I had just gotten robotics and had like 6 mobile art and 3 mech inf. Moral of the story, drafting and nationalism can save ur ass.

I use nationalisim whenever I have either a spy economy or a civ that has a gunpowder unit as it's UU. Drafting is awesome. The extra spy boost comes right soon after the economics obsoletes castles, so i like that a lot.

nationalism kicks ass for situations like yours where you need troops asap. even better if have some gold to spare and can buy and draft same turn (guess its possible to even combine with slavery and alternate the whip with buying although usually emancipation required for the happies).

my biggest war problems are the smiles, so hereditary rule/polic state , nationhood are great combos. the AI is relatively dumb even on high difficulty settings so playing well you can hold or take cities with massive deficiencies of troops

Drafting can be powerful, and buy you enough time if you have failed to keep military strength.