@BalrogDeMorgoth Setting aside how this is clearly a chemical fire (water made it worse) which may very well be impossible for the enemy to understand without the MC there to explain it to them, given how severely he was outmatched in terms of an army, including how they managed to overcome a lot of his modern technological advantages despite having the element of surprise so firmly in his hands, there's a good chance that if he held back hoping to save something for the
next fight, he probably wouldn't have gotten the opportunity to live to see said next fight.
Even if we work under the assumption that if he successfully repels this attack they'll just come back and attack later with counter-measures to his defenses (since MC doesn't know that the country will fall apart if they don't succeed in this war effort), he will have a lot of time to setup proper defenses. He had begun laying down some good inroads to getting actual reinforcements, setting up good defenses, and being able to actually deal with this sort of attack without needing to bring the technological equivalent of a shotgun to a knife fight, with the time it'd take them to mount another war-level invasion attempt he should be able to fight them on a more equal level since he won't be coming in shortly before the invasion while lacking the funds and manpower needed to fight them off.