if Castro was a Communist at first. Some folks that knew him from his student days (like an uncle of mine), claim he was, because he was a Communist himself! But that means little, lots of students were Reds in the 30s and 40s. The Comintern was active all over the world, and the University of Havana was filled with working class kids studying on government scholarships. In Cuba, tuition was free as long as you could pass your examinations, which were very competitive. You can't let working class kids go to college, they're way too idealistic and they get into all sorts of trouble.

Castro was mostly known as a bonchista, a member of a "bunch", a term borrowed from English for politically organized radicals, opportunists and troublemakers of both Left and Right. There was even a Right-Wing Catholic bonche!

Whether he became a full-fledged Marxist-Leninist due to prior conviction or merely to ingratiate himself with his Soviet masters in the 60s is still a matter of debate. At any rate, it really doesn't matter.

It is often said, as you imply, that the US drove him to the Soviets solely because of our inflexible opposition to his nationalizing of private industries and businesses, and particularly because of his Agrarian Reform program which broke up the big estates and landholdings. But this may be an oversimplification of a much more complex evolution.

All we know for sure is he publicly declared full Marxist principles AFTER he completely liquidated the old USSR-dominated Cuban Communist Party and all other Socialist, Left-wing and Anarchist groups. Until then, the average Cuban really had no clue as to his alleged Communist connections. Essentially, he played us off against the Soviets, and they won. We might have kept Fidel out of the Soviet orbit if we had met him half-way, but no one really knows for sure.

My own opinion, (and that's all it is, an opinion) is that Castro would have preferred a middle-of-the-road, Socialist government, but he felt obliged to break up the big landholdings, and the big industrial interests, many of which were American-owned. But this was not possible in the late 50s and early 60s. Even the slightest hint of "nationalization" was immediately interpreted as Communism by the Americans, and would not be tolerated. Castro knew he got lucky during the Bay of Pigs. He knew the next time, the Yanquis would come in force.