Congress tried to make serving in a foreign military cause you to loose your citizenship, but that was struck down by the supreme court. The actual case where it was struck down involved a guy with dual US/Israeli citizenship voting in Israel, but the law that was struck down was the same one that tried to punish serving in a foreign military.

Now, keep in mind that it might still be illegal (with most likely an exception written in for Israel), but at least you won't loose your citizenship for it.