Just finished The Imperial Cruise, by James Bradley (Flags of our Fathers, Flyboys). It is one of the most remarkable examples of what a good historian does that I have ever read.
Bradley's father was one of those depicted raising the flag on Iwo Jima in the most iconic representation of the Pacific War, 1941-1945. The Imperial Cruise undertakes to reveal some of the events at the turn of the last century that led to Japan's mid-century assumption that it could colonize with impunity about one-third of the land in the world.
The short answer to that is that in 1905, Teddy Roosevelt (the same one whose bust is one of the quadpod that disfigures a peak in the Black Hills) dispatched a delegation to Japan that "informally" told the Emperor of the "Honorary Aryans" (TR's characterization of those whom he privately affectionately termed "Little Jap Monkeys") that the USA would give Korea to him if the Japanese would "civilize" Asia.
Japan proceeded to try to do so, using as a template the United States' model for civilizing Hawaii, Cuba and, especially, the Philippines. Sure is annoying when you give one country another whole country and they end up turning on you thirty years later. Nearly as annoying as when your country undertakes to affectionately torture and massacre a people into assimilation and they resent you for it.
Had to bitterly grin to myself as I reflected on the US's handing over of Korea (the right of ownership of which was assumed by TR by virtue of his Teutonic/Aryan lineage) to Japan. Kinda like what happened to me a third of a lifetime ago when a lawyer assumed ownership of something of mine and gave it to someone else. Like Korea, I simply didn't have enough firepower to effectively dispute who owned my property.
WWI, WWII, the Korean War, the Vietnam War, and even our current Iraq/Afghanistan quagmire all have roots in events described in The Imperial Cruise.