I became friends with someone on line and he claims to be in Afganistan in the service. I think he took the identity of a serviceman and is n Nigeria. I have not been scammed but to marry him.
You've made no friend. You're in contact with someone using a persona. The US Army and the FTC are way ahead of you in anticipating what the persona was created to accomplish.
Online Romance Scam Information - U.S. Army Criminal Investigation Command
[excerpt]
Army CID is warning anyone who is involved in online dating to proceed with caution when corresponding with persons claiming to be U.S. Soldiers currently serving in Afghanistan or elsewhere.
Army CID receives hundreds of allegations a month from victims who state they got involved in an online relationship with someone, on a legitimate dating website or other social media website, who claims to be a U.S. Soldier. The "Soldier" then begins asking for money for various false service-related needs such as transportation costs, communication fees, marriage, processing and medical fees. Victims of these online scams have lost tens of thousands of dollars, with a very low possibility of recovery.
FTC Warns Consumers About Online Dating Scams - Federal Trade Commission