I'm copying an answer I wrote a while back:

The Japanese did become more radicalized. While it's true that the attack on Manchuria/China was simply an extension of their previous wars of conquest. Attacking the US and the UK at once certainly wasn't the logical conclusion to their colonial empire. Neither was the way in which the conquest was conducted. For instance, in WWI the Japanese were cited by the Red Cross for their exemplar care of POWs while in WWII mortality rate in Japanese POW camps was the highest of any country in the war IIRC.

To make a comparison, Germany was more radicalized in WWII even though the war they fought in 1939-1945 was similar strategically to the one they fought in 1914-1918 (both entailing conquest of large parts of the Russian/Soviet Empire as a part of a two front war etc). Similarly, Japan was more radicalized even though the wars it fought was in part a logical continuation of the previous colonial wars they fought. The Japanese government went in the interbellum from what amounted to a sort of a democracy to a full-blown out autocratic government during the war. The conquests they made were explicitly based something which was similar to the Lebensraum ideology the Nazis justified their conquest with. Japan was allied with the UK in WWI and wouldn't have even thought of starting war with the UK/US at the same time. The way they viewed the Chinese and the other peoples they conquered in the same way the Nazis viewed Slavs. The treatment of Koreans also got dramatically worse during the 30s and 40s. In the end, there was no equivalent of the rape of Nanking in WWI.

So why did they become radicalized? Let's start with the year 1919. The answer is actually remarkable similar to why the Germans and Italians were radicalized. First of all, Japan felt cheated at Versailles, most of all, it felt humiliated when it introduced a clause at Versailles calling for acknowledgement of racial equality in the world and got it thrown back into their face by the other nations there. This is similar (though for other reasons) to the resentment Italians felt with regards to the "mutilated victory" and the Germans regarding the Versailles "diktat". It also made the Japanese overcompensate for the humiliation by seeing themselves as the "Master Race" of Asia.

The other probably even more important answer, like in the German case, is the Great Depression of the 1930s. The depression first of all discredited liberal Capitalist Democracy in Japan, which seems unable to resolve the economical crisis. This turned the Japanese people against the Taisho democracy of the 1920s. This era also planted the idea that wars of expansion and the settlement of conquered territory with "Yamoto blood" (and other, similar racial ideas) as the solution to economical woes. It also drove right-wing terrorism from within the military against the civilian government, which was perceived as corrupt and weak and the cause for suffering of millions of Japan's poorest.

Which brings us to the radicalization of the military in the 20s and 30s. And how it became a nearly autonomous entity which basically dictated Japan's foreign policy. Besides the mentioned depression, there was also the Washington and London Naval Treaties, which, signed by Japan's government, limited the size of Japan's Navy and which the military saw as leaving the country unable to defend itself. Which further turned the military against the civilian government. Which manifested itself both as the military started to act increasingly independently and in attacks on the civilian government.

The acts of terrorism from within the military culminated in the assassination of Japanese Prime minister Inukai Tsuyoshi in 1932, and the attempted coup d'etat of Feb 26 1936. Those acts cowed both the Japanese civilian government and the emperor himself. The Japanese Kwantung army, acting in collaboration with the industrial conglomerates called Zaibatsu (which controlled much the economy) was basically conducting it's own foreign policy without input from Tokyo, most notably in the war in Manchuria and later China. In a manner far more brazenly aggressive than Japan's civilian government ever was before, in terms of disregard for international opinion, the brutality of the conquests, and the size of the territory taken.

Which brings us to the last point, which takes place in the late 30s and early 40s, which was the war in China itself. The war in China consumed resources like oil at a massive rate, the US then enacted an oil embargo against Japan in 1940 in response to the Japanese occupation of Indochina (which itself was due to the idea that China was being supplied by the west through Vietnam). The Japanese had no domestic source of oil, and its stocks would run out in a couple of years, which meant it had to accept "defeat" in a war which was already a stalemate in China. So at this point, Japan faced the prospect of either war with the UK/US/Holland to seize oil producing territories (like the Dutch East Indies), or decline in power in the future. And thus why the Japanese launched their most radical war ever.

TL;DR: The Japanese did become more radicalized after WWI, it was because of the Great Depression, a humiliation at Versailles, the Washington Naval Treaty, and the War in China further radicalized an already radicalized Japan

Source: Nail Ferguson: War of the World

American Shogun, General MacArthur, Emperor Hirohito and the Drama of Modern Japan by Robert Harvey