Do you need the dates? I am not a pro of historical dates :(.
Early 19th century: Japan is under a very strict isolationist policy. The country is basically locked down in order to protect the Tokugawa shogunate's supremacy. The Japanese culture is flourishing, but the technology is about the same as when Japan started its isolation, near 1600 (basically, katanas and muskets)
1866: American Admiral Matthew Perry asks the Shogun to reopen the country by entering Tokyo's harbour with heavy warships. Intimidated by the size of these ships (the Japanese must've felt like if the aliens from Independence day showed up and asked the president to "trade" with them). A year later, the Shogun ended the Japanese isolationism.
Late 19th century: The influence of International traders and visitors shakes the Japanese social structure. The Samurai class, namely, sees these reforms as a menace to their aristocratic power and privilege. Some of them believe that the Shogun is too keen on adopting the European's reform, and try to re-instate the Emperor, the religious leader of shinto, as the ruler of Japan. A few civil wars later, the Emperor is the new leader of Japan (Meiji Restoration). But the Japanese realizes that, if they wish to still be major players in this more interconnected world, they will need to get their military and technology up to date.
Even later 19th century: Japan basically absorb 200 years of technological progress in 30 years. Japan quickly adapts to the modern tactics and weapons, and starts building an empire by invading small Pacific Island and Korea. They later invade Manchuria, were they get in a struggle with Russia for the domination of the territory. A war broke out in 1904-1905, and Japans comes out victorious. This is very important, since it shows that Japan has the potential to rival a European power, which is surprising to the colonizing Europeans.
WW1: Japan profits from WW1 to invade and conquer some Pacific Island belonging to the Germans by joining the Entente. They join the League of Nation, truly defining them as an international player.
1930's: The Great Depression hits Japan's economy pretty hard. Japan decides to redress it's economy by strengthening their military and trying to build an empire spanning across the Pacific. They leave the League of Nation because of their War Mongering.
Pre-American WW2: Japan expands its territory by invading China and a lot of Pacific Islands. This War-Mongering starts to stress the American, which holds territories, protectorates and states under their influence in the regions (Guam, Hawaii, the Philippines...) decides to stop sending oil to Japan. Since the US sent almost 80% of Japan's oil, the Japanese war effort is effectively slowed down. In their militaristic thinking inherited by spending a lot of the last 80 years fighting, having a feudal leader as their head of state and being confident on their capacity of defeating an occidental nation, they try to intimidate the US by attacking the base of Pearl Harbour. This mesure did not only failed to intimidate the US, but encouraged them to break from isolationism and join WW2 to exact vengeance. So you could say Pearl Harbour's attack kinda backfired
TL;DR: Japan turned into a militaristic powerhouse in a very short amount of time while having a feudal leader. This turned them into a very war-like nation during the 20th century.
I'm sorry if my orthograph/syntax/english in general is not perfect, French is my native language.
