Hemingways_Shotgun

I feel like the answer’s in your question…

It’s not a colonialist threat. It’s effectively an act of war.

I’d have to go with Virus.

Wipes out enough people to give humanity a chance to at least try something new from the ashes of the old world, but doesn’t leave a lingering threat like Zombies or Aliens or something like that. The survivors (ostensibly those immune to the virus) have struggles, for sure, but at least can rebuild without worrying about a constant existential threat.