Bounty hunters will come. If first swarm won't get the bounty it will be even worst, as a new swarm will come after the first.
Fighting against a friend is a net loss. But less than with a real fight against a real opponent. Let's make a (not real but) easy example.
Nation A have a bounty on him.
Nation B is a friend of A, nation C an enemy.
Let's say that both B and C would made and suffer same ammount of damage (quite sure C would do more than B, but go on assuming they do the same).
At the end of the war with A against his friend B, A get beiged.
A suffered 50m of damages, B suffered 50m of damages.
B takes the bounty and the loot. He keep the bounty and some part of the loot to go even: so he keeps for himself 50m.
He than gives back to A 20m. A suffered 30m of damages, B 0 damages.
Now do A against C. A get beiged.
A suffer 50m of damages. C 50m.
C takes the loot and the bounty: so C have a net earn of 20m.
A have a loss of 50m.
The math is easy, first war to the alliance means 30m damages, second war to the alliance means 50m damages.
Numbers are surely incorrect, but I think I have explained well my idea on the bounty system