Japan is known for being isolationist, so what cultivated its history of interventionism in the 20th century?

After ending isolationism, Japan modernized. It built guns, ships, and planes. Then it started invading places under the Emperor's orders. The invention of a chemical process that made gunpowder cheap may also have contributed. Also, I would not call Japan "interventionist" so much as aggressively imperialistic.

Industrialization required access to resources that are not readily available on the Japanese islands. Japan didn't just decide to be a colonial power, they decided to be an industrial nation as well. That's the key.

Today, we have free and open markets, so buying raw materials on the open market works. Prior to WW2, however, if you needed a steady supply of material from a faraway place, you needed to achieve political or military control of that place to guarantee that supply.

In 1859, Charles Darwin published "Origin of Species," which postulated a world of "survival of the fittest," in which animals, at least, had to "eat or be eaten." That is, nothing lived in isolation, but rather in a Hobbesian world of "all against all."

Coincidentally. or not, international relations of the late 19th century assumed that kind of flavor, whereby whole COUNTRIES had to be colonizers or colonized. Faced with this choice, Japan elected to be a "colonizer," with relatively easy targets nearby in Asia, including Korea, Formosa (now Taiwan), China, and Indochina.

The "gun was pointed" at Japan when America's Admiral Perry stopped by in 1854 and pulled Japan into what was then the "modern" world.

After the Meiji restoration, Japan wanted to model itself on the western colonial powers. So, it sought an colonisation and expansionist policy as was common for Western powers of the time. By the early 20th century, colonial empires were already set and Japan had to invade other countries to expend. They chose to do that instead of remaining a small nation which could be conquered themselves. In addition, conquest of the land of Asia was always on the table -- remember the (failed) Korean invasions of 1592-1598.

The end of Japan's isolation wasn't voluntary - Japan was forced out of its isolation by American gunboats sailing into Tokyo Bay.

Basically, they saw the world being divided into colonizers and the colonized, and decided they'd rather be the former. (Despite being theoretically closed to all outside contact before then, Japan did get some news from the outside, including descriptions of the Opium war in China.) Military modernization was a high priority from the beginning.

Without going into too much detail, in the late 19th/early 20th centuries, they colonized Korea and a few others, and jockeyed with America and European countries, including Russia, for influence in China. That was just how the international game was being played at that time.

Then they were hit especially hard by the Depression of the 30s. They depended a lot on export trade, much more so than America. Japan wasn't nearly as rich, either financially or in natural resources, so you can imagine how much worse off Japanese workers were when things hit the fan.

With the other developed countries putting up trade barriers to protect their own workers, Japan decided it was important to have a self sufficient economy, which meant controlling markets and natural resources in more of Asia.

As in Germany, hard conditions at home made foreign invasions an easy sell. Factions of the military stirred up old resentments against Japan's rivals, told the people that complete submission to a military regime was the only answer, and next thing you know it's WWII.

That's a lot of details omitted, but it's a pretty big subject :P.

Japan shut out the West very successfully so its emergence from isolation was all the more abrupt, and Japan's history to 1945 could be seen as trying to integrate Japan's self-image and national mythologies, and its powerful social factions, into a post-feudal, industrial state. And quickly!

The Japanese leadership made a quick but effective plan to survive integration with the world, in a way that would avoid what they saw happening to China and preserve as much as possible of Japan's old order. This meant taking Western technology and acquiring great power status so as to negotiate all matters from a position of strength. Empire-building was an obvious characteristic of a great power, and Japan went about acquiring an empire. They quickly deduced there were only two sides to imperialism, and they determined to be the ones sticking flags in the ground.

Because the Europeans (and US) could negotiate directly with one another, they mostly avoided conflict when building their empires. Japan, an outsider could not, and as a latecomer was seen as a threat. The Japanese saw this resistance as hypocrisy and racism, which is why relations were good up until the 1920s, then soured.