Pearl Harbour killed 2,403 Americans [0].

We're at 500,000 now with the virus, I think?

That's more Americans than have been killed in all the 20th Century wars combined [1].

[0]https://en.wikipedia.org/wiki/Attack_on_Pearl_Harbor

[1]https://en.wikipedia.org/wiki/United_States_military_casualt...

By that sort of reasoning, when a nation rolls tanks into your borders, you should just let them do it because more people would die if you fight back.

If a nation actually did that (release a bioweapon and pre-immunize their own citizens)... well World War is probably overselling it, but I could certainly imagine contained conflicts, sinking of cargo vessels, shooting down of planes, targeted assassinations... etc. It's very likely that every nation would have highly vested interests in making sure that whoever authorized that weapon was removed from this planet.

The reasoning wouldn't be how many people died of the virus vs. how many in a world war; it would be how many people die in a world war vs. how many might die if the adversary does increasingly worse things going forward. The threshold for fighting all-out can't be "as soon as the adversary does".