Every March 31, the U.S. Virgin Islands of Saint Thomas, Saint John and Saint Croix observe “Transfer Day” to commemorate the sale of the islands from Denmark to the United States. Of the U.S.’s five permanently inhabited territories, the U.S. Virgin Islands is the only one the country ever purchased from another imperial power. The two powers negotiated over the three islands for more than 50 years before finally transferring power in 1917.

Though the U.S. and Denmark each had their own complex motivations in this exchange, “they turned upon the question of imperialism—declining in the case of Denmark and increasing on the part of the United States,” wrote the late historian Isaac Dookhan in a 1975 issue of Caribbean Studies. Ultimately, the U.S. would successfully pressure Denmark to sell the islands by threatening a military attack on the neutral nation during World War I.

Denmark had colonized the three islands—known as the Danish West Indies—back in the 17th and 18th centuries. It forced enslaved Africans to work on plantations producing products like sugar, which it profited from until the 1840s, when sugar prices fell.

Big changes also came in July 1848, when several hundred enslaved people on St. Croix revolted and won their freedom by threatening to burn the islands’ towns to the ground. After abolition, these newly freed people struggled to make a profit on exhausted lands and plantations that were small and old-fashioned compared with newer industrial operations, according to the Danish State Archives.

By the late 19th century, Denmark was finding it increasingly expensive to run the islands. Yet as early as the American Civil War, the U.S. was eyeing them as a possible economic and national security asset. This was because U.S. officials thought the islands could help secure American economic interests in the Caribbean. But they also worried a hostile foreign power might take control of them before the U.S. could.

“During the 1880s and 1890s, suspicion was directed mainly against Germany, which was developing interest in Latin America,” Dookhan wrote. “The fact that the German steamship company, the Hamburg-American Line, used St. Thomas as its regular refueling station tended to exacerbate those suspicions.”

The first negotiations between the U.S. and Denmark began in 1865, the year the Civil War ended. Secretary of State William Henry Seward actually negotiated a treaty with Denmark to cede the islands to the U.S. in 1867, but the Senate rejected it. This was probably partly due to an anti-expansionist sentiment that set in after the Civil War, and partly due to the fact that the Senate was angry at Seward over his support of President Andrew Johnson during his impeachment trial, notes the U.S. State Department.

Negotiations started up again in the 1890s but fizzled with the onset of the Spanish-American War in 1898. In the aftermath of that war, the U.S. gained the territory of Puerto Rico in the Caribbean and the territories of Guam and the Philippines in the Western Pacific (Puerto Rico and Guam are still U.S. territories; the Philippines won independence after World War II).

READ MORE: How the United States Ended Up With Guam

The U.S. was a larger imperial power now, with a greater interest in expanding. It had also set its sights on building the Panama Canal, and this made it even more interested in purchasing St. Thomas, St. John and St. Croix in order to secure the future canal’s route. Again, another secretary of state (this time John Hay) negotiated a treaty with Denmark. The Senate ratified the treaty in 1902, but this time, the Danish parliament rejected it.

In 1915, the fear of German takeover motivated the U.S. to make another try for the islands. Especially after the sinking of the Lusitania, President Woodrow Wilson and Secretary of State Robert Lansing feared that Germany might annex Denmark and launch more attacks from the Danish West Indies. Danish leaders resisted ceding the islands and their majority-black inhabitants to the racially-segregated United States.

Angry at this, Lansing insinuated that if Denmark didn’t sell the U.S. the islands, it just might go and seize them to prevent Germany for getting to them. It was a bullying tactic, and it worked.

Eager to prevent a U.S. military attack (Denmark was currently a neutral party in World War I), Denmark negotiated a treaty with the U.S. that President Wilson signed on January 16, 1917. On March 31, 1917, Denmark formally transferred governance over the islands to the U.S., and the U.S. reciprocated by paying Denmark $25 million in gold coin.

What this meant for people living on St. Thomas, St. John and St. Croix—now the U.S. Virgin Islands—was unclear. In 1920, the acting secretary of state specified that Virgin Islanders had “American nationality” but not the “political status of citizens.” This changed in 1932 when Virgin Islanders won American citizenship, but voting was a separate battle.

The U.S. Virgin Islands didn’t win the right to vote for their own governor until 1970. Today, American citizens in the U.S. Virgin Islands—as well as those in the U.S. territories of Puerto Rico, Guam, American Samoa and the Northern Mariana Islands—still cannot elect voting members to Congress or vote for the president of the United States.

READ MORE: Puerto Rico’s Complicated History with the United States