What if the Carnation Revolution which overthrew the almost-fascist government of Portugal went to the Communists. In OTL, leftist officers tried to overthrow the officers who overthrew the Caetano government, but the effort fizzled. What if the communist leaning troops were better organised, or their opponents less so.
In 1975, Portugal and Spain were considered the backwater of Europe. Portugal was saved from being at the very bottom of the stack by Albania, and Spain wasn't much better off. How would the rest of Europe react to a Red Portugal?
To give this TL a proper point of departure, lets have General Spinola, the leader of the coup and generally conservative, taken out in a plane crash shortly before the original coup takes place. Without his influence, more leftish and outright Communist members of the Armed Forces Movement gain position and influence.
Spain is preparing for a post-Franco government. The rest of Europe is somewhat center-leftish and not particularly interested in intervening in anything. The US is still grappling with the post-Vietnam malaise.