What nation in the Western Hemisphere, right on "America's doorstep," became pro-Communist in 1959?

The answer is Cuba!

Cuba became pro-Communist in 1959

Explanation:

Because of the Spanish war, Cuba's control was passed to Spain through America. The American administration, therefore, governed it. During this period, most public officers were filled with Cubans who worked under the Spanish rule. Much was done during this period that was notable such as education improvement, sanitation, and eradication of yellow fever.

Through this provision, Cuba promised not to incur further debts that the current revenue will not contain while continuing with the clean sanitation that was undertaken by the American Military.