The Spanish–American War was a short decisive war marked by quick, overwhelming American victories at sea and on land against Spain. The Navy was well-prepared and won laurels, even as politicians tried (and failed) to have it redeployed to defend East Coast cities against potential threats from the feeble Spanish fleet. The Army performed well in combat in Cuba. However, it was too oriented to small posts in the West and not as well-prepared for an overseas conflict. It relied on volunteers and state militia units, which faced logistical, training and food problems in the staging areas in Florida. The United States freed Cuba (after an occupation by the U.S. Army). By the peace treaty Spain ceded to the United States its colonies of Puerto Rico, Guam, and the Philippines. The Navy set up coaling stations there and in Hawaii (which voluntarily joined the U.S. in 1898). The U.S. Navy now had a major forward presence across the Pacific and (with the lease of Guantánamo Bay Naval Base in Cuba) a major base in the Caribbean guarding the approaches to the Gulf Coast and the Panama Canal.
Try to answer this question if possible (otherwise reply "unanswerable"):  What island territory voluntarily joined the UK as a colony in 1898?
unanswerable