Camilo Mejia, 28, born in Nicaragua and raised in Miami, was given a bad conduct discharge and sentenced to a year in jail on Friday.
He was convicted by a special court martial at Fort Stewart army base near Savannah, Georgia.
Last October, while on a two-week home leave, Mejia refused to return to his outfit in Iraq, saying the war was "illegal."
Call of conscience
He asked the military for conscientious objector status, was refused, and was charged with desertion instead.
In March, military officials said Mejia would be tried by a special court martial rather than a general court martial, sparing him the risk of facing a death sentence.
Todd Ensign, one of Mejia's lawyers, said the conviction was "a miscarriage of justice."
In Nicaragua, Mejia's father, Carlos Mejia Godoy, was bitter over the sentence.
The United States, he said, "uses its big stick against anybody who tries to think differently".