Members of my race can live virtually infinite lives (they still have to drink, eat, and can be killed, but don't age and are immune/really resistant to disease) but are unable to reproduce.
They used to be "manually" created by a god, who has not manifested since hundreds of years ago, thus not giving birth to any new member of this race ever since.
However I feel like this applies to pretty much any long-lived-but-slow-at-reproducing race (such as classic Tolkien-like elves), so the matter is more general:
Why would a long lived race decide to wage war and fight others, knowing that (regardless of the outcome) they will be losing many more people than they will be able to give birth to in a long time?
This seems pretty common in fantasy, but I feel like such a race would be much more sensitive towards even the smallest losses, since their low fertility makes extinction a potential risk.
I want to know what good reasons this race would have to enter a war or generally have high aggression (not only the obvious ones like being forced to choose between a defensive war and annihilation), because I would like the race to be fairly warlike despite the risks.
To be clearer about what I mean with "having a reason to fight": for example, one idea might be having the god create them with an innate instinct to fight (like Orcs and Half-orcs in D&D, for example) but I'm afraid that would sound too cliché. I don't want them to be utterly self-destructive, since they would go extinct in a matter of years, just prone to and good at fighting.