A police squad, acting under orders from Governor of Puerto Rico Blanton Winship, opened fire on demonstrators protesting the arrest of Puerto Rican Nationalist leader Pedro Albizu Campos, killing 21 people and injuring 235 others.

Blanton C. Winship was an American military lawyer and veteran of both the Spanish–American War and World War I. During his career, he served both as Judge Advocate General of the United States Army and as the governor of Puerto Rico. An investigation led by the United States Commission on Civil Rights blamed him for the Ponce massacre, which killed 19 people.

The Ponce massacre was an event that took place on Palm Sunday, March 21, 1937, in Ponce, Puerto Rico, when a peaceful civilian march turned into a police shooting in which 17 civilians and two policemen were killed, and more than 200 civilians wounded. None of the civilians were armed and most of the dead were reportedly shot in their backs. The march had been organized by the Puerto Rican Nationalist Party to commemorate the abolition of slavery in Puerto Rico by the governing Spanish National Assembly in 1873, and to protest the U.S. government's imprisonment of the Party's leader, Pedro Albizu Campos, on sedition charges.

The Nationalist Party of Puerto Rico was a Puerto Rican political party founded on September 17, 1922, in San Juan, Puerto Rico. Its primary goal was to work for Puerto Rico's independence. The Party's selection in 1930 of Pedro Albizu Campos as its president brought a radical change to the organization and its tactics.

Pedro Albizu Campos was a Puerto Rican attorney and politician, and a leading figure in the Puerto Rican independence movement. He was the president and spokesperson of the Nationalist Party of Puerto Rico from 1930 until his death. He led the nationalist revolts of October 1950 against the United States government in Puerto Rico. Albizu Campos spent a total of twenty-six years in prison at various times for his Puerto Rican independence activities.