FORT DRUM, N.Y. — After graduating from the United States Military Academy at West Point near the top of his class in 2008, Second Lt. Lawrence J. Franks Jr. went on to a stellar career with three deployments, commendations for exceptional service and a letter of appreciation from the military’s top general.

The only problem: None of it was in the United States military.

After being sent to Fort Drum, here in the snowy farmland of northern New York, where he was put in charge of a medical platoon, Lieutenant Franks disappeared one day in 2009. His perplexed battalion searched the sprawling woods on the post for his body.

What they did not know was that he was on a plane to Paris, where he enlisted under an assumed name in the French Foreign Legion. It was only this year when he turned himself in that the Army and his family learned what had happened.

On Monday, Lieutenant Franks was sentenced to four years in prison and dismissal from the Army on charges of conduct unbecoming of an officer and desertion with the intention to shirk duty, specifically deployment.