The answer to that question is the estimated 20,500 "non-U.S. citizens"--serving in all branches of the U.S. military.
Indeed, the first U.S. service member killed in the Iraq war, on March 21, 2003, was Marine Lance Cpl. Jose Gutierrez, a native of Guatemala.
Active-duty non-citizens who have served honorably in war on or after September 11, 2001, may "file for immediate citizenship," according to the Defense Department.
Nearly 37,000 non-citizens of the U.S. armed forces have been granted citizenship since the war on terror began in October 2001; 109 have been granted posthumously, according to the U.S. Citizenship and Immigration Services.
Another 7,300 still have their requests for citizenship pending.
Historically, we have had immigrants in the military dating back to the Revolutionary War.