The one success for German diplomacy in this period of time was the reorganization of Eastern Europe. The Russians had seized - or tried to seize - virtually every inch of territory that had been put in their line of interest in the deal with Hitler. And in 1940, they seized control of part of Romania. The end result of that was to scare the living daylights out of the Romanians, and basically force them into an alliance with Germany. the Romanians were so scared of the Russians, that they even agreed turning over Transylvania to Hungary, which is where it should have been to begin with, but which the Hungarians were cheated out of after World War I.
But this was a fairly reasonable reorganization of boundaries, and is probably one of the few redeeming aspects of German foreign policy within this period. The rest of what happened was just a series of disasters, and a study in confusion. The biggest disaster was planning to attack the Soviet Union without having defeated England. This was just a complete impossibility. There were just not the resources to do this. The reasons for doing that, we will discuss later. In early 1941, the British and the Americans encouraged a coup in Yugoslavia, against German interests. Hitler retaliated by not only invading Yugoslavia, but by cleaning up the mess the Italians made on invading Greece. And the end result was that German influence extended through Yugoslavia to Greece, in fairly short order. This, however, further drained German military resources before an attack on the Soviet Union.