Why is Hawaii a part of the USA?
In 1819, King Kamehameha II became King of Hawaii. His father had conquered some of the other islands and united the chain under his rule. The new king, however, began his reign by abolishing a form of paganism which involved idol worship and human sacrifice.
Into the somewhat chaotic void thus produced came a man named Hiram Bingham, a Protestant missionary from New England, who preached the gospels and brought with him a group of other missionaries. Catholic missionaries arrived a few years later, and after considerable sectarian strife religious toleration was established.
In 1823, Kamehameha II and his wife visited Great Britain, and died there. (I'd like to know how this happened, myself.) This left the claim to the throne contested, and the monarchy weak. While this was going on, an American firm called Ladd & Company started the first sugar plantation on the island of Kauai. By 1840, Hawaii had a constitution modelled on that of the US, and this country, Britain, and France recognized Hawaii as an independent, sovereign state.
Queen Lillioukalani came to the throne in 1891 and tried to assert a more vigorous monarchial government, but two years later American troops landed and assisted an American-inspired revolt which toppled the queen. Sanford Dole was made the first (and only) president of the brief Hawaiian republic which followed the revolution.
American business interests dominated the new government, and by 1898 they succeeded in getting Congress to annex the republic. Despite repeated attempts to make it happen sooner, most notably at the efforts of President Truman, Hawaii was admitted as a state in 1959, the fiftieth of the fifty states.
(I got most of this from the World Book Encyclopedia, so it's not just off the top of my head.)