Well even that would have required first some incentive on behalf of the west, say round up all the reds and shoot them. And why would that happen?
That's not really at all far-fetched. In the 30's, the US deployed tank and machine-guns to intimidate striking longshoremen. On several prior occasions, the National Guard had been used to kill strikers.
National Guard tanks on the waterfront (Photo: San Francisco History Center, San Francisco Public Library)
National Guard machine gun nest (Photo: San Francisco History Center, San Francisco Public Library)
So it's not unreasonable to assume that the US would have reacted to a Communist revolution in Italy, or anywhere else in Europe, in much the same way that it reacted to Vietnam: deploy troops to prevent the next "domino" from falling.