Other than Pearl Harbor, this is the key move that helped bring down the Axis.
The necessity of dealing with the Greek resistance and subsequent British intervention (the necessity being to protect the all-important Ploesti oilfields), took vital time away from the Axis forces - slowing down the onset of Barbarossa and also mildly denuding the forces available for its success.
The failure of the invasion of Russia, which indeed could have been a success, was the deathknell for Nazism.
Another massive factor in its failure was - Pearl Harbor. If the Japs had attacked Russia, as their army faction were minded to do, then Kutuzov could never have brought his Siberian troops back in time to defend Moscow. The USSR would have collapsed and any eventual D-Day would have faced vastly more potent tank forces.
Instead the Japanese Navy launched the militarily brilliant, but strategically toxic attack on Pearl Harbor. Kutuzov was free to move his forces, and of course the sleeping giant awoke.
If Hitler had our hindsight, he would have cursed Yamamoto every bit as much as he cursed Mussolini.
Great Britain could not afford to be lax on that front for fear that Italy could flip to hostile at any time. Hitler would have been able to avoid having to rescue Mussolini from his own idiocy in Greece and North Africa keeping one his best tank generals (Rommel) on the eastern front.
Funny that nobody knew it at the time though: The later failure in Crete, in Greece itself were all in the future. And nobody was seeing the long-range impact on the Russian invasion the next summer.
the Allies were lucky that Musso was such a pompous pig-headed blundering fool
no way were the Italian armed forces prepared to conquer Greece while heavily engaged in North Africa
this was an impulsive decision of ego and pride, predicated on wishful thinking for immediate collapse of Greek resistance
the Italian army was only given a handful of days to prepare for the invasion and did not have nearly the forces to conquer Greece if resistance was prolonged (as it was in fact)