Did the US help the partisans ? would require a pretty long answer, the short one is yes they helped "some" partisans, though I believe the support was more British than US.
There were a number of "bands" in Yugoslavia, with some actually fighting on the side of the invaders, and Tito initially was just one if the many, Mihailovic being the other "big one". Tito showed remarkable skill on uniting very different "cultural" groups towards the common goal of fighting the invaders and limiting inter band fighting.
Up to the decision by all the allies to back Tito and drop Mihailovic it was by no way a sure thing he would have come out as the dominant figure after the axis defeat, it's quite possible Yugoslavia would have had it's civil war in 1945 not 35 years later. To go back to the initial question stating the communist "exploited" the partisans post war is incorrect, the partisans, as they existed at war's end, were led by Tito and he had always been a communist, so they would be exploiting themselves.
Any attempt to describe the situation in Yugoslavia in a few lines is hopeless, initially there was a disbanding army that with some units deciding to "go to the mountains" and continue the fight rather than turn in the weapons to the occupation forces. Then there were multiple ethnic groups and religious distinctions as well as ideological ones, and most of them gave birth to some sort of armed formation that were not very prone to collaborate with each other. There were also two different occupation forces (Italy and Germany), with conflicting long term goals, and each was ethnically tied to significant local minorities, three major supporting allies (USSR, GB and USA) each with it's own agenda and initially supporting different local forces, and finally there was also a deposed king.
Just to make things more interesting the Germans also brought in Cossack troops, and that meant not just the soldiers but also their families that attempted to settle in the areas allocated to them, and after the September armistice 1943 a very large number of Italian troops found themselves stranded in Yugoslavia with no way of going back home as the Germans controlled the roads.
Edited by TiredOldSoldier, 13 January 2016 - 06:54 AM.