Hawaii was a territory of the United States. With an appointed Governor, territorial legislature, a representative sent to Congress though it was non-voting, and was very much under the control of the US. The kingdom of Hawaii had been overthrown in 1893 by a movement mostly managed by powerful US commercial, and planter interests on the islands, then after a few years pretending to be independent was formally annexed by the US in 1898.

And essentially from the start it was developed as first a stop over point and coaling station for US warships(along with more commercial flow) focused on Honolulu and Pearl Harbor on the island of Oahu. By 1941 the vast majority of residents on the island had spent either their entire lives, or most of it as a place formally owned by the US.

By 1941 it was the primary anchorage of the US Pacific Fleet, having moved from California the year before. It was the home port of the majority of the United States Navy, including almost all its battleships and most of its newest ships and large Army and Army Air Force units were also present on the island. These forces were the target of the Japanese attacks not the islands themselves.

It served a similar function to any of the major forward bases of the Royal Navy over the years like Gibraltar, Malta, or Singapore, thus was a very attractive target if the practical difficulties could be worked out.

Hawaii was a territory of the United States during the time, having been annexed by the United States in 1898. While it retained some self-governance as a territory, the large islands at the southeastern part of the chain became (in fact, were at time of annexation) valuable as land for sugar plantations controlled by the "Big Five" companies that, essentially, ran Hawaii as an oligarchy.

The anchorage at Lahaina Bay on the west coast of Maui, as well as the large harbor of Pearl Harbor in Oahu, were seen early on by the United States Navy as logical places for the fleet to exercise itself, and as the likelihood of war with Japan grew throughout the early decades of the 20th century, Pearl Harbor grew into a major naval base. In particular, US planners built a large dock and dry dock facility there, developed Ford Island in the middle of the harbor into a joint Army-Navy base, dredged the harbor entrance and areas for ship berthing, and developed the naval shipyard.

In May of 1940, partly in response to tensions with Japan, the US Pacific Fleet (at the time, based at San Diego) was ordered to take an "advanced position" at Pearl Harbor. It remained there until the attack of 7 December 1941, when most of the battleships of the fleet were damaged in the attack (luckily, the US aircraft carriers were away, and the port facilities were not seriously damaged).

I wrote more about the attack here, if it's of interest. Please let me know if you have follow-up questions!