It postponed Barbarossa , that's a fact, but I believe those troops would have to be diverted anyway and had the Italians not attacked the British may have taken a chance later at an even more crucial point for the Axis. In a way by letting do the Germans do the job, this area was no longer a threat . this region was too close from Austria. The real trouble came later when partisan troops prevented several division from gaining the east front by keeping them busy with sabotage and guerilla. I a way Mussolini's blunder allowed hitler to seize not only Serbia but also Greece and therefore cause trouble to the Brirish in the eastern Mediteranean.

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