From the link I posted above:

"Although a person's enlistment in the armed forces of a foreign country may not constitute a violation of U.S. law, it could subject him or her to Section 349(a)(3) of the Immigration and Nationality Act [8 U.S.C. 1481(a)(3)] which provides for loss of U.S. nationality if an American voluntarily and with the intention of relinquishing U.S. citizenship enters or serves in foreign armed forces engaged in hostilities against the United States or serves in the armed forces of any foreign country as a commissioned or non-commissioned officer."