In November 1940, Hitler brought Romania and Hungary into the Axis alliance, primarily because he needed to cross them to get to Greece and, later, the Soviet Union. Bulgaria joined in March 1941. Yugoslavia, however, refused to follow, so Hitler ordered an invasion, which began on April 6, 1941.

On April 10, German forces began attacks on Belgrade from the northwest, north, and southeast. The city fell three days later, quickly followed by the surrender of the Yugoslav army. Like many other conquered countries, however, Yugoslavia proved more difficult to hold than it had been to take. Guerrilla fighters would continue to harass the Nazi invaders over the course of the war.

Next, Hitler went after Greece, which was better prepared to fight than Yugoslavia had been. Its fighting force of 430,000 men was fully mobilized and battle-tested, but things went poorly from the start. The Greeks' efforts to defend the Metaxas line northeast of Salonika proved to be a disaster, and German troops forced the surrender of the line and nearly half the Greek army on April 9. Almost two weeks later, the Greek First Army was attempting to pull out of Albania but became trapped at the Metsovon Pass and was forced to surrender.

The German drive accelerated after that, with the Nazis taking the Isthmus of Corinth by April 27 and the Peloponnesus by April 30, forcing British defenders into an evacuation that cost nearly 12,000 men. In addition, a week-long airborne assault in late May brought Crete under German control.

As all this was going on, German general Erwin Rommel had launched a very successful counteroffensive against the British in Libya, pushing them from the country with the exception of an isolated garrison at Tobruk. North Africa was quickly falling into German hands.