If you have limited resourced (no money), but you do have military power, you can invade a neighbor with resources. If a neighbor invades you, you defend yourself.
Neither of those positions is irrational, except maybe the first (maybe), but there's certainly scenarios where it would be your only choice.
Mutually assured destruction on a planetary scale does come about rather early in a civilization's history (if we're assuming galactic, or beyond-galactic scales of civilization). But that doesn't really mean it is easy to come about on a galactic or universal scale. Is an enemy destroying a planet or solar system you control really such a big deal when you're battling to control a third of the universe? Even having an entire galaxy destroyed would be like an enemy destroying someone's living room on an earth-scale.
Of course, this is all just making guesses about technology way beyond our current comprehension. Maybe universe-destroying bombs come up rather early in technology on this scale. Who knows.
I read somewhere there's been something like 3000 wars in recorded human history. Seems like a pretty frequent thing.
Thru out human history, raiding weaker neighbouring villages for resources, or access to females would be pretty rational behaviour for your villages growth and continuing success of your close genetic relatives.
So much so that parts of Human Male's anatomy seems to be purposely driven for combat and survival. Things like larger muscles, thickness of skull, thickness of neck. Also, societies seem to come together and cooperate, hunker down, in war and conflict. Also seems to me to have some genetic component to that social behaviour.
Only about 50% of men have reproduced through out human history. Raiding a neighbouring village, killing the man, and taking the best women for wives would be a rational though unethical behaviour. Its exactly what the Vikings did.
In fact, there's a growing concern that large and growing percentage of men, currently unattractive to females (internet incels) will lead to radicalization and possibly conflict in the west. As those man try to better their odds of reproduction. Currently they are genetic rejects.