It was the second time that an American service member had been killed in Iraq since President Obama resumed military operations there nearly two years ago.

In the days after Sergeant Cardin’s death, American military officials were forced to disclose why he and the Marines were at the base, how Marines would be used in the future and how many American troops were actually in Iraq. The new information illustrated how the conflict had quietly expanded far from the public’s view, and raised questions about Mr. Obama’s pledge to keep American troops out of combat there.

“Just because the commander in chief says there won’t be combat doesn’t mean that will be the case,” said Sergeant Cardin’s brother, Vincent Cardin, a former Army infantryman, in a telephone interview. “It doesn’t take much for someone to launch a rocket and start a fight when you’re in someone else’s country. If that’s not combat, I don’t know what is.”

From the beginning, Sergeant Cardin’s mission in Iraq was secret.

He was assigned to the Second Battalion, Sixth Marine Regiment, part of an expeditionary unit of roughly 2,200 Marines based on three Navy ships in the Persian Gulf. Last October, Sergeant Cardin left Camp Lejeune, N.C., for the passage across the Atlantic and the Mediterranean, and through the Suez Canal and the Strait of Hormuz.

By November, he and the Marines had arrived in the Gulf, where they remained aboard the ships as a quick-reaction ground force in case of a crisis nearby. They were to return to Lejeune this spring.