Wikipedia states that several sources, including Hitler himself, believed that the German invasion of Greece delayed Operation Barbarossa, preventing German forces from achieving outright victory initially and beginning a war of attrition on the Eastern front. Did the Battle of Greece actually have this effect?
Note: I could only find one instance where a question of this type was asked before, and it had no replies.
The German diversion to Greece did not cause delays that had much of an impact on the success of Operation Barbarossa. The Soviet Union had a very limited network of paved roads. The Moscow Smolensk highway was the only intercity road in the USSR in 1941 that had a hard surface. Most freight moved by rail or on barges. There is also a season that the Russians called Rasputitsa, when the snows melt and the frozen ground thaws out. This turns the dirt roads of Russia into a sea of mud. It is very difficult to move around in Russia during May and the first half of June, because of this mud. Any invasion of Russia before June 22nd, would have bogged down quickly.
Napoleon Bonaparte invaded Russia on June 24th 1812. His army was primarily an infantry army, with horse drawn cannons and some cavalry. Yet Napoleon was able to capture Moscow. He marched his army straight towards that objective. After the Germans won the battle of Smolensk, they did not drive straight towards Moscow, but diverted the two Panzer Groups that had been attached to their Army Group Center, south to envelop Kiev. This helped Army Group South to win a major tactical victory and capture some 680,000 Red Army prisoners. However, it deprived Army Group Center of six weeks of good weather and stalled their drive to Moscow.
Source : "When Titans Clashed: How the Red Army Stopped Hitler" by David Glantz & Johnathan House