Well, the invader is the last party to want a war: they'd be happy to just take over.

Russia's takeover of Crimea in 2014 was almost as 'peaceful' as the German takeover of the Sudetenland in the 1930s.

All in all, my comment is just a convoluted way that sometimes the price to pay for peace is too high.

In addition, there's also pre-commitments that make everything more complicated: as a potential victim of invasion, you might want to pre-commit to defending long beyond any reasonable threshold, in the hopes that this will deter invasion. Sometimes your bluff gets called, and then you need to actually fight to maintain your credibility.

Compare mutual assured destruction in nuclear war: nuking Moscow in retaliation for the Soviets nuking New York isn't going to bring anyone back from the dead. But it's what you pre-commit to in order to deter the bombing of New York in the first place.