Guam is home to the sprawling Andersen Air Force Base and the Naval Base Guam. The Pacific island, which Ashton Carter, President Obama’s defense secretary, called “an important strategic hub for the U.S. military in the Western Pacific,” is on the Pentagon’s plans for expansion—even if the move might not be popular with many of the 160,000 people who live on Guam.

Guam is home to 6,000 U.S. troops, a number that is expected to double over the next decade. The Pacific island’s location is also pivotal for the U.S. It’s midway between the restive Korean Peninsula and the South China Sea, where China has territorial disputes with many of its neighbors, some of which are U.S. allies. North Korea’s threat to strike Guam not only hits at the heart of U.S. interests in the region, but also strikes at the core of the insecurity felt by those Guam residents who want the U.S. to leave.

Guam’s relationship with the U.S. is complicated. It’s a U.S. territory, or as Admiral Robert Willard, the former head of the U.S. Pacific Command, told lawmakers in 2010, it’s “the farthest west U.S. territory that we own.” Still, Guam’s residents are U.S. citizens—just like those people who live in the 50 states, as well as Puerto Rico and the Virgin Islands, even if they cannot vote in presidential elections on the island.

The island’s link to the U.S. was cemented after the Spanish-American War of 1898, when it was transferred to the control of the U.S. Navy. It remained a possession of the U.S. until December 1941 when the Japanese invaded and occupied it two days after the attack on Pearl Harbor. The Japanese occupation was brutal—about 1,000 people were killed—and lasted nearly three years. In 1944, the U.S. recaptured the island in a bloody battle against the Japanese.

The island has been a U.S. territory since then. Threats against it—this time by North Korea—could force the U.S. to revisit that bloody history.