Did the entrance of the United States into World War II, make the defeat of Germany, Japan and Italy inevitable?
Inevitable, no. Very likely, yes. Many things could have happened in the war that would have shifted the tide of the war in a different direction, though outright Axis victory seems very unlikely. It was hardly preordained, for example, that the Allied landing at Normandy would come off successfully, or that the German advance would be halted at the Battle of the Bulge. Similarly, the American victory at Midway was very closely run, and had the United States not been able to decipher the Japanese naval code, it might have been the US carriers entering an ambush rather than the Japanese. Other events out of the control of the Americans also could have influenced the course of the war. The Germans, for example, could have been successful in their invasion of the USSR by capturing Moscow or Stalingrad, both of which they were very close to accomplishing at various points. But to look at the question a different way, it is highly unlikely that the Allies could have defeated the Axis powers without American entrance into the war.