At this time preparations for Undertaking Barbarossa continued. But Hitler's immediate aim was the closure, by Undertaking Marita, of the open door to Britain offered by the continuing war in Greece. Since January the build up of German forces in Romania had convinced the British government that an invasion of Greece was imminent; and when the capture of Tobruk (on 21st January, 1941) had made Egypt secure, Churchill proposed to divert part of the British army there to Greece. On 24th February the Greeks accepted. At the same time, British policy aimed at winning over Turkey and, particularly, Yugoslavia, whose support might well turn the tables on the Italians in Greece. But the Turks were reluctant and the Yugoslavs were under heavy pressure from Germany too. On 4th March the Yugoslav government, following the example of Bulgaria, decided to adhere to the Tripartite Pact of Germany, Italy, and Japan. On 25th March, in Vienna, the Yugoslav Prime Minister and Foreign Minister signed the Pact with Hitler. Greece was thus isolated, and the way was open for Operation Marita.

                But at this moment a well planned revolution in Yugoslavia reversed all. On 26th March a group of Serb nationalist officers, who were known to be in revolt against the pro German policy, overthrew the Regency and the government in the name of the young king, and set up a new government of clearly anti German character. Hitler received the news first with incredulity, then with indignation. Summoning the High Command, he declared that the new Yugoslav government could not be trusted, that its unreliability would jeopardise both Marita and Barbarossa, and that he was resolved to take no risks. He would destroy the country militarily and as a national unit. No diplomatic inquiries will be made, no ultimatum presented. Yugoslavia was to be pulverised with merciless brutality ..... in a lightning operation. On the same day he issued his Directive No. 25.