If you serve in the military, you definitely are a citizen.
Not in the USMC. Foreign nationals can join but there is no guarantee of citizenship in exchange for service. We had Afghans in our Company being trained to fight the Russians with the mujahedeen. Definitely not US citizens then or now.
This official US government website says you need to apply:
https://www.uscis.gov/military/naturalization-through-military-service