Hi there Devil Dog,
Well, they were a staunch ally in the Great War, losing 15% of their male population in that one, and they were again our allies in WWII, losing a million people to the nazis and rescuing about 600 American airmen in Operation Halyard:
http://en.wikipedia.org/wiki/Operation_Halyard
I do know that historically they have been our allies in WWI. In fact, their little country is the reason for WWI. However, during WWII they became a Nazi puppet state so there were partisans but there were also a lot of Serbians on the Nazis side. However, all of this history is not relevant.
What is relevant is if they have recently been an ally of ours. They were communists up until 1992 and since then as an independent nation they have been allied closely with Russia, not the U.S. So the answer is no they have not been our ally recently which was the point I was making.