Actually, Master Chief and the Spartan program wasn't started to fight in the alien war.
It was originally an effort to suppress rebellion against the UNSC. Master Chief fights for the government that kinapped and brainwashed him into being a child soldier.
Realistically the Spartans should defect to the covenant out of pure spite. They have more in common with elites than they do humans anyway.
@Chickiepup somebody write this game
I wanna say it's in one of the books. I know a lot of the Spartan IIs end up subverting the UNSC.
Those sad kids just wanna be free.
Jace's personal Mastodon instance