Why are the us marines in Afgan?

it might sound like a dumb question but is it because the Taliban and the terrorists? or is there a different reason? and is this war worth dying for? because it is not for our freedom please help I'm thinking about enlisting in the corps but i don't want to die for a country that does not want us there if it was for WWII there would be no question about it i would go 0311 any help would be nice thanks