Which branch of military is on the front line?

Answer 1

All of them. Air Force and Navy pilots penetrate the furthest into enemy territory than anyone else. The Air Force has their Tacp, Pararescue, and Security Police personnel, who are often in harm's way, and it's much the same with units such as the Navy's Construction Battalions, not to mention that, as the Marine Corps does not have certain services (such as a medical corps), those services are provided by Navy personnel, who will be attached to Marine units. Air strikes are called in by Air Force Tacp personnel. So it's not just the Army and Marine Corps.

Answer 2

Marines will fight anywhere in the world, every longitude every latitude. The army sustain ground combat.