When a great evil arises, shouldn't you fight it?
I would argue no. Avoiding great evil will always be preferential to engagements which entangles oneself in their wake of destruction.
Now, in reference to your example. One should be willing to defend against a looming danger.
Let us for example contrast North Korea, Saddam's Iraq & Nazi Germany. All undoubtedly ruled by evil.
I struggle though to accept the question of degree of evil as the justified reasoning for a moral imperative to fight or avoid.
In fact, the morality of attacking Germany and not North Korea or hypothetically reconsidering the 2003 invasion of Iraq to me comes down to degree to which there was indeed the seeds of reformation. Germany emerged a different beast, both America and Germany the better for it.
Contrast this to Iraq or Afghanistan. I'd consider neither particularly reformed. No doubt less hellish, but still hellish countries none the less full of great evil. Contrast this still to other post-war countries like Rwanda, far from reformed, but better. America for her noble quest, certainly is not the better for the 2003 Iraq war...(btw, I was pro-war att)
So why not go fight the evil of North Korea even when failing to due so with Nazi Germany would have been equally sinful.
It's simple.
All evil is potentially dangerous, but that is not enough, avoid and only when there is a looming threat should one fight, but this being an important caveat. When you fight, leave no remanent nor peacetime sentiments, fight with absolute zeal.
It might be noted, that is said, as exampled by the fact NK, ISIS, Saddam's Iraq fo 2003 etc etc are all evil directly allowed to stand and fester because those who fought for the moral good failed to be absolute in their victory.