When 1941 began, German Chancellor Adolf Hitler was reaching the peak of his authority in Europe. Only Great Britain opposed him. Because he believed that country was too weak to be a direct threat, he felt free to shelve Operation Sealion, the invasion of England, and direct his attention east. On Dec. 18, 1940, he issued Directive No. 21, better known as Operation Barbarossa, the invasion of the Soviet Union. The plan called for the invasion to commence on May 15, 1941. Instead, the invasion was launched six weeks later, on June 22 – a delay that ultimately proved fatal.

There were two reasons for that delay. The more famous is Operation Marita, Germany’s invasion of Greece to rescue the failing Italian invasion of that country. Though Marita did have an impact on Barbarossa because of its use of armor, it was not as great as is generally assumed. In fact, Hitler had from the outset factored in German army assistance to Italy. As Barbarossa called for simultaneous attacks from the Baltic Sea in the north to the Black Sea in the south, Hitler secured alliance treaties with Hungary, Romania and Bulgaria that in addition to letting him position troops for Barbarossa, allowed him to transport troops for Marita. Yugoslavia, though neutral, was an important holdout regarding the latter.

“The bulk of the tanks that came under me for the offensive against the Russian front in Southern Poland had taken part in the Balkan offensive, and needed overhaul, while their crews needed a rest.”

It is Hitler’s diplomatic failure with Yugoslavia, which resulted in Operation 25 – the invasion of Yugoslavia on April 6, 1941 – that is less recognized for its impact on Barbarossa. Though a relatively small German army was deployed in Operation 25 and the subsequent Greek campaign, it contained a disproportionate number of tanks. As Ewald von Kleist, who would lead a panzer army in Barbarossa, later said, “The bulk of the tanks that came under me for the offensive against the Russian front in Southern Poland had taken part in the Balkan offensive, and needed overhaul, while their crews needed a rest.”

So, why did Hitler reroute vital panzer units assigned to Barbarossa for a strategic sideshow instead of securing a diplomatic solution? The truth was, he had reached a diplomatic agreement with the Yugoslavian government – and it blew up in his face.

At the time Yugoslavia was a monarchy ruled by the regent Prince Paul on behalf of the young King Peter II. Despite overwhelming popular support to remain neutral, with neighboring Hungary, Romania, and Bulgaria already in the German camp, Prince Paul felt he could no longer hold out. A secret meeting between Hitler and the prince was held in Germany on March 4-5, 1941, and a deal was struck. On March 25, the Yugoslav premier and foreign minister secretly arrived in Vienna and signed the treaty.

When Yugoslavia’s nonaggression treaty with Germany was announced the next day in the Yugoslav capital of Belgrade, Prince Paul and the government were promptly overthrown in a popular uprising led by a number of Yugoslav Air Force officers with the support of the Yugoslav Army. On March 27, King Peter II was officially installed as king, ending the regency.

Shouting that he had been “personally insulted,” Hitler demanded that Yugoslavia be crushed with “unmerciful harshness and that the military destruction be done in Blitzkrieg style.”

Hitler, meanwhile, was congratulating himself over his diplomatic victory with the Yugoslavs. Then, at five minutes before noon on the 27th, as he was readying himself to meet the Japanese foreign minister, he received a telegram from Belgrade informing him that the ministers who had signed the treaty had been jailed and a new government had been installed. At first Hitler thought the telegram was a joke. His disbelief quickly changed into what was later called “one of the wildest rages” of Hitler’s life.

The meeting with the Japanese foreign minister was postponed. Hitler called for a conference with his top military leaders and Foreign Minister Joachim Ribbentrop – one ordered so hastily that some arrived late. Shouting that he had been “personally insulted,” Hitler demanded that Yugoslavia be crushed with “unmerciful harshness and that the military destruction be done in Blitzkrieg style.” As for Barbarossa, he announced: “The beginning of the Barbarossa operation will have to be postponed up to four weeks.” It was a passage underlined in the top-secret notes taken of the meeting.

Historian William L. Shirer wrote, “This postponement of the attack on Russia in order that the Nazi warlord might vent his personal spite against a small Balkan country which had dared to defy him was probably the most catastrophic single decision in Hitler’s career.”