For two years after the Spanish-American War, the United States operated a military government in Puerto Rico until 1900, when the first civilian government was established. The governor, his cabinet, and the senate-like Higher House of Delegates was appointed by the U.S. president. A 35-member Local House of Delegates and a resident commissioner, who represented Puerto Rico in the U.S. House of Representatives but had no vote, were elected by popular vote. In 1917, Puerto Ricans were granted U.S. citizenship by President Woodrow Wilson.

Living conditions in Puerto Rico advanced very little in the first 30 years under U.S. rule. A couple of hurricanes between 1928 and 1932 left the economy—dependent solely on agriculture—in ruins. Homelessness and poverty were rife. The unhappy state of affairs fueled the organization of another independence movement led by the Harvard-educated nationalist leader Pedro Albizu Campos. The doctor chafed against U.S. rule and asserted it had no claims to the island because it had been given its independence from Spain before the Spanish-American War broke out.

A gifted orator, Campos traveled throughout Latin America garnering support for Puerto Rico’s independence and was named president of the island’s Nationalist Party, which had formed in 1922. In 1935, four Nationalists were killed by local police under the command of a Colonel E. Francis Riggs in an event referred to today as the Río Piedras Massacre. The next near, Riggs was killed in retaliation by two Nationalists, who were arrested and executed without a trial. Campos was arrested for his suspected role in the death. The first jury trial found him innocent, but a second trial found him guilty and he was sentenced to prison.

On Palm Sunday in 1937, a Nationalist Party demonstration was organized in Ponce, Campos’s hometown, to protest the independence leader’s incarceration. Just as the march was getting under way, police fired on the crowd, killing 19 people and injuring 200 in what went down in history as the Ponce Massacre. It was a huge blow to the independence movement, and with Campos imprisoned, it seemed as though the fight for freedom had been quelled. Instead, the incident merely drove the movement underground and possibly fueled its embrace of violent tactics.

To quell the brewing unrest, protect its interests, and benefit from the island’s resources, the United States took several momentous steps beginning in the 1940s that had far-reaching effects on Puerto Rico’s culture. During World War II, several large military bases were established on the island—Fort Buchanan Army Base in Guaynabo, Ramey Air Force Base in Aguadilla, Roosevelt Roads Naval Station in Ceiba, and Vieques Navy Base—which significantly boosted the economy. In 1940 a major hydroelectric-power expansion program was undertaken, providing electric power throughout the island and attracting U.S. industry. In 1947, President Harry S. Truman agreed to give Puerto Rico more control of its local government, and the next year the island chose its first self-elected governor, Luis Muñoz Marín, a member of the Popular Democratic Party.

But by this time, Campos had finished serving his time and returned to Puerto Rico, where he reinvigorated efforts to achieve independence—this time, at any cost.

On November 1, 1950, two Puerto Rican nationalists—Oscar Collazo and Griselio Torresola—attempted to assassinate President Truman at the Blair House, where the president and his family were living while the White House was being renovated. Approaching the house from opposite sides, they attempted but failed to shoot their way in. After the gunfire ended, Torresola and one police officer were dead, and two police officers were wounded. Collazo was sentenced to death, but Truman commuted the sentence to life. Campos was again arrested and found guilty of his role in planning the assassination attempt. He spent the remainder of his life in and out prison until his death in 1965.

In 1952, Puerto Rico adopted a new constitution, and Commonwealth status was established. The island had more self-governing powers than ever before. This was the beginning of the long debate that still rages today over Puerto Rico’s political status. While roughly half the population is content with Commonwealth status, an equal number of residents have worked steadily toward trying to achieve statehood.

Puerto Rico’s first self-elected governor, Luis Muñoz Marín, was a New Deal–style reformist with progressive ideas who served four terms as governor of Puerto Rico. In partnership with the United States, he initiated many programs that advanced economic and cultural development throughout the island and significantly improved the infrastructure. Under his leadership, an economic development program called Operation Bootstrap was successfully launched to entice global industry to the island with federal and local tax exemptions. The Economist described it as “one century of economic development…achieved in a decade.” The standard of living leapt to new heights, and the tourist trade soon exploded. The next three decades, from the 1950s through the 1970s, was a huge period of growth and development for the island. But some believed Operation Bootstrap was a throwback to colonial ideals in which the island’s resources were exploited without fair compensation, rendering the island increasingly more dependent on the United States.

The island suffered several setbacks in the 1980s. The energy crisis and U.S. recession sent the tourist trade into decline, and many of San Juan’s glamorous high-rise hotels fell into disrepair, some shuttering altogether. Hurricane Hugo in 1989 dealt a devastating blow, and Operation Bootstrap was discontinued, which sent many manufacturers packing.

Meanwhile, the independence movement was quietly gaining momentum, and peaceful protest was not part of the agenda. Two pro-independence organizations formed in the 1970s. The Popular Boricua Army, commonly known as Los Macheteros, primarily operated in Puerto Rico. The Armed Forces of Puerto Rican National Liberation (FALN) operated in the United States. The two organizations communicated their desire for independence with terrorist attacks.

One of FALN’s most notorious attacks was setting off a briefcase bomb in 1975 in New York City’s Fraunces Tavern, a historic landmark where George Washington delivered his farewell speech to colonial troops during the Revolutionary War. Four patrons were killed. Other bombs were detonated in a Harlem tenement, Penn Station, and JFK Airport. All told, FALN set off 72 bombs in New York City and Chicago, killing five people and injuring 83.

In 1981, Los Macheteros infiltrated the Puerto Rican Air National Guard base and blew up 11 military planes, causing $45 million in damage. In 1983 members of Los Macheteros raided a Wells Fargo depot in Hartford, Connecticut, wounding a policeman and making off with $7.2 million, ostensibly to fund the organization’s efforts.

Sixteen instigators from both organizations were eventually captured and sentenced to federal prison, bringing the terrorist acts to a halt. In 1999, President Clinton granted them clemency.