After reading up on president McKinley (I got curious after Obama renamed his mountain), I eventually started reading about the US annexation of Hawaii.
A brief summary is that a white American minority with the help of the US military overthrew the native Hawaiian monarchy and established a brief oligarchic republic with the hopes of getting annexed by the USA, which they eventually were in 1897 and this was done against the wishes of the native Hawaiian people.
American expansionism in the late 1800's is not surprising, as they went to war with Spain and crushed the Philippine revolution around this time. What IS surprising is why they took Hawaii when they did: the Japanese. The Japanese were one of the premier military powers in the pacific at this time and with the exception of Britain had a navy to match any contender in the region. Hawaaii has numerous military and trade applications as proven during WW2, so it makes sense that both Japan and America wanted it. There seems to have been a genuine fear at the time that Japan would annex Hawaii, so the US had to get there first, as Japanese control of Hawaii could prove a serious impediment to US power projection into Asia. These fears were far from unfounded either, as the Japanese sent a battle-cruiser to Hawaii (along with another warhsip that was already in the area) as part of their diplomatic protest against the US annexation. Obviously nothing serious came of this, but it spooked the American government. One ship was a threat, but if the Japanese had sent an entire flottila they likely could have seized the islands as they were already far more established there than the Americans were.
It seems that the Hawaiian monarchy was being heavily courted by the Japanese government and more importantly a census in 1900 shows that almost 40% of the population was Japanese! Compare this to the white population, which was less than 20% of the total population and was not solely American either. There appears to have been a very real chance that the Japanese could have gained effective control of the islands, either through an alliance with the native monarchy by promising protection in exchange for military ports etc, or outright annexation. To give you an idea of just how close the two were, the King of Hawaii even proposed a royal marriage to bring their two nations together. The proposal was rejected, but Japanese-Hawaiian relations went from strength to strength.
https://en.wikipedia.org/wiki/Japanese_in_Hawaii
In terms of justification the Japanese would have had plenty to gain from such a move and the USA at the time was far from the mighty behemoth it would eventually become by the time the two countries went to war in WW2. Add to this the fact that the USA went to war with Spain around the time of annexation, you have to wonder what might have happened had tensions between Japan and America been more tense. A Japanese-Spanish alliance perhaps?
Heres am article discussing it in more detail, although take it with a pinch of salt as it is very pro Japan, particularly towards the end as it claims the Japanese were very peace loving at the time compared to the US, despite the fact they had just recently kicked the crap out of China!
http://www.iiipublishing.com/blog/20...5_25_2009.html
It should also be noted that the British and French were none too pleased with the annexation either, the British in particular harbored their own ambitions for influence in Hawaii, although they were almost certainly far too overstretched with their existing possessions to seriously commit to anything. That said the status quo of a neutral, free Hawaiian nation suited them much better than a Hawaii controlled by the USA, allowing for increased American influence in the Pacific.As a result, were Japan and America to go to war, it is unlikely that the British would have intervened and could possibly have offered some indirect support to Japan in an attempt to curb American ambitions.