Editor's note: This is the first of a two-part series on "non-citizens" serving in the U.S. military.
TWENTYNINE PALMS, California (CNN) -- U.S. Marine Lance Cpl. Mario Ramos-Villalta flashes a broad smile from beneath his camouflage cap as he talks about the country he loves and why he became a Marine.
Mario Ramos-Villalta of El Salvador has served two combat tours in Iraq, but has yet to get U.S. citizenship.
He's fought twice in Iraq and survived an attack on his Humvee in October 2005. Now, he's preparing for deployment to Afghanistan.
Yet he's not even an American. He's a citizen of El Salvador serving in the U.S. military.
"A lot of the papers I get [say], 'You're a great American,'" the 22-year-old Purple Heart recipient says. "I am not an American citizen yet, but I still fight for it."
He adds, "Sometimes, I do get depressed about still not being a U.S. citizen and going over there."
Ramos-Villalta isn't alone. He is one of an estimated 20,500 "non-U.S. citizens" -- dubbed "green-card warriors" -- serving in the military. See photos of sacrifice and his Purple Heart »
In fact, 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 who came to the United States when he was just 14. Ramos-Villalta says it fills him with pride that Gutierrez was an immigrant just like him.
"It shows that even though he wasn't an American, he stands on our side and he goes over there and he dies for that reason."
Platoon commander 2nd Lt. Benjamin Brewster offers similar praise about Ramos-Villalta, saying he's a dedicated Marine who has put his life on the line for his comrades and for America. He says those sacrifices should not go unnoticed by the U.S. government.
"He understands the freedoms that he enjoys in this country -- and I would say at a level that lots of Americans don't," Brewster says. Watch Ramos-Villalta describe getting attacked in Iraq »
The United States has tried to make it easier for foreigners serving in the military to become citizens. After the September 11, 2001, terror attacks, President Bush signed into law a measure allowing active-duty non-citizens who have served honorably in war on or after September 11, 2001, "to 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, which works closely with the Pentagon.
Another 7,300 still have their requests for citizenship pending, says Chris Rhatigan, a spokeswoman for U.S. Citizenship and Immigration Services. It takes about seven to 10 months to process an application, she says.
"These service members have made extraordinary sacrifices for our nation and we're going to do everything possible to ensure that qualified immigrants who serve in our military and who wish to receive U.S. citizenship receive that at the absolute earliest opportunity," Rhatigan says.
She adds, "We have had immigrant members of the military going back to the Revolutionary War."
Mark Krikorian, the executive director for the Center for Immigration Studies, says Americans should keep watch on the Pentagon to make sure it doesn't go beyond the current program.
"My concern is not that the current situation is a problem, but that it could grow into a problem if the Pentagon gives into the temptation of using citizenship -- or even an offer of a green card -- as a way of enticing non-citizens to enlist," says Krikorian. His Washington think-tank advocates less immigration, but better treatment for those who are admitted.
In the case of Ramos-Villalta, he says it's been a painstaking process because he's been deployed to war; he was wounded in action; and is now training for a third combat deployment, leaving him little time to deal with the paperwork and lawyers needed to file for citizenship.
"It's frustrating and sometimes I get real sad about it," he says. "There is nothing I can do about it. I mean it's not up to the military. It's up to Immigration Services."
Rhatigan says her agency is doing the best it can to make sure military members know where they can turn to get help. They've established a hotline, (877)247-4645, and set up a Web site at www.uscis.gov/military to help better inform these immigrants.
"We're there for them," she says.
Ramos-Villalta fled El Salvador's civil war in 1989 with his family as a young boy and lived in the States for a time. He returned to El Salvador with his mother, while his father stayed in California to work. He eventually made it back to Southern California and got his green card when he was 13.
He graduated from Santa Ana High School in 2004, becoming the first in his family to graduate from high school.
His 17-year-old sister, Ivette Ramos-Villalta, beams when she talks about her brother's military service. "He's fighting for a country that is not even his, but because he grew up here he keeps fighting because it's the country he actually loves," she says.
What would it mean to Ramos-Villalta if he gets U.S. citizenship?
"It would make me feel great about myself. I think I do deserve to be a U.S. citizen, and I would actually fight for my country knowing I'm an American fighting for my country." E-mail to a friend
CNN's Gregg Canes and Traci Tamura contributed to this report.
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