Which territory was annexed by the United States in 1898, contributing to the start of the Spanish-American War?

The correct answer is Hawaii, as its annexation in 1898 played a significant role in the lead-up to the Spanish-American War, though the relationship is a bit nuanced. The annexation of Hawaii provided the United States with strategic military and economic advantages in the Pacific, particularly for facilitating naval operations. The islands served as a coaling station and a way station for ships traveling to and from Asia, which was increasingly important for U.S. interests.

Meanwhile, the actual conflict with Spain in 1898 emerged from different issues, such as the struggle for Cuban independence and the explosion of the USS Maine in Havana Harbor. However, the U.S. involvement in Hawaii not only set the stage for American imperial expansion in the Pacific but also reflected the broader context of American interests shifting towards overseas territories, which became pivotal during and after the war.

As for the other territories mentioned, Puerto Rico and Guam were indeed acquired as a result of the Spanish-American War, making them closely tied to the war's immediate aftermath rather than the events leading to its initiation. Alaska was purchased in 1867, well before this period, and thus does not fit into the context surrounding the Spanish-American War.