At one point, Yossarian explicitly says that the enemy is anyone who would get him killed. For the most part, he's referring to his own administration, and not the Germans. Colonel Cathcart and Colonel Korn have especially vindictive grudges against Yossarian for subversive actions that they take rather personally. Colonel Cathcart even says that he hopes Yossarian gets killed on a mission so that it will relieve the administration of its problem. Cathcart keeps raising the number of required combat missions, which is a pretty darn antagonizing act.