No.
If you serve in another countries military, you should loose your citizenship automatically.
8 U.S. Code § 1481 - Loss of nationality by native-born or naturalized citizen; voluntary action; burden of proof; presumptions
(a)A person who is a national of the United States whether by birth or naturalization, shall lose his nationality by voluntarily performing any of the following acts with the intention of relinquishing United States nationality—
(1)obtaining naturalization in a foreign state upon his own application or upon an application filed by a duly authorized agent, after having attained the age of eighteen years; or
(2)taking an oath or making an affirmation or other formal declaration of allegiance to a foreign state or a political subdivision thereof, after having attained the age of eighteen years; or
(3)entering, or serving in, the armed forces of a foreign state if (A) such armed forces are engaged in hostilities against the United States, or (B) such persons serve as a commissioned or non-commissioned officer; or.....
So all the people who served in the Lafayette Escadrille in WWI; the RAF’s Eagle Squadron and the American Volunteer Group (the Flying Tigers) in WWII should lose their citizenship?
Free men are free to fight for causes they believe in. The only exception is those who take up arms for an enemy against our country like some German-American Bundists during WWII and Islamists who joined ISIS or Al Queda.