In 28 October 1940 Italians launched a surprise attack against Greece across the greek-albanian borders... After some initial success their advance halted and greek army launched his counter-attack and pushed them back...
By early spring 1941 Italians were retreating having lost about 1/3 of Albania when Hitler attacked Greece in 6 April 1941 after calls for help by Mussolini...
WI Hitler had denied to help Mussolini in his struggle and spare his troops for Operation Barbarossa?
Hitler is more successfull in Russia?
Mussolini would keep fighting or he would negotiate a peace treaty with Greece?
knowing about the entente of 1917, hitler needs to bring this area in his hands.
so i has to attack
only a early italian victory could avoid it - or no attack at all.
In this case the germans come in deep trouble if the brits invade greece (basicly they did in 1917) and try to disrupt german oil support.
Sure, greek patisans and maybe the army will fight but the big question is "how long"?
If the germans attack one month earlier (this will not happen cause of bad weather, 2 weeks is the best) things get interested. the brits are way to weak to hold greece even against low german forces... in 1941/42 they are no enemy in such position (they need to tranport the whole stuff by sea (german airforce has plenty airfields), greec partisans try to disrupt british forces... german army, if concentrates, will beat any british army in 1941/42. The british losses will be brutal, a secon dünkirchen, just without the ships to rescue the troops.
For hitler, this could be better as historically...
a funny ah...

"last stand at mykene", the way 500.000 british troops get captured, 200.000 dead, 300.000 prisoners.
This caused also the quick loss of malaya and australia...
yes - really interesting and fun.