The attitude of the Japanese Empire since the Meiji restoration has been expansionistic in nature to various degrees. The 1910s to the late 1920s were a period characterised by the Taisho Democracy where there was an increase in civilian power within the government. Emperor Taisho was a sickly man and thus most of the power within Japan fell into the hands of the Japanese Diet (the parliament). This civilian dominated government generally opted for peaceful coexistence and cooperation with other powers through trade and agreements. This was supported by the Anglo-Japanese Alliance formed in 1902 which came into effect during WWI which is why the Japanese sided with the Entente powers. The Japanese stood to benefit from this arrangement as they relied on the British to support their naval expansion since they Kongo-class Battlecruisers were being constructed by Vickers and they would potentially be able to capture German colonies for themselves.

After the end of WWI, the Japanese came out of it much like Italy. They were victors in the war but got almost none of the spoils. The Japanese were able to obtain German possessions such as Shantung in China but were still disgruntled as they felt their contributions were not recognised by the Western powers. The League of Nations that was formed afterwards still had racist undertones as the western powers refused the Japanese request to add a clause to the covenant stating that all men are equal. At the same time, the militaristic factions began to assert themselves with the death of Emperor Taisho and the coronation of Emperor Showa which was more receptive to their expansionistic plans. This was further compounded by the Washington Naval Conference where the Japanese Navy was restricted to 60% of the strength of the US and UK highlighting their inferiority. The Washington system was an attempt to establish a new world order for international security where it would be ruled by international convention rather than Bilateral agreements. The British while keen to renew the alliance with Japan to potentially stop their expansion into British territories were ultimately forced to let the alliance lapse due to American pressure to ratify the Washington Naval Agreement. Thus at this point, the Japanese had been humiliated on the international stage and militarist-nationalist factions were on the rise in Japan.

The Great Depression struck Japan hard as it decimated their export industries. This led to the Japanese attempting to expand into Manchuria in order to achieve autarky which upset the status quo in the region. The US and UK wanted the status quo in China and the Far East to stay the same as they benefitted from an open and weak China where no power was dominant. Japanese expansion threatened to upset this and establish their dominance which was fundamentally against the interest of the British and the Americans. The British who were in no position to really intervene did attempt to apply some level of diplomatic pressure through the Lytton Commision and their report on the Japanese invasion of Manchuria. This subsequently led to the Japanese leaving the League altogether which only further deteriorated relations between the two. The Japanese gradually formalised the doctrine of Nanshin-Ron which was the expansion of the Japanese Empire into Southeast Asia and the Pacific. This put them into direct conflict with the Allied powers who wanted to maintain their colonial possessions which eventually led to war as their interest were inherently not compatible.

Overall, the cause of the change in the Japanese was both due to a rise in militarism within the empire itself which emphasised the need for economic independence and national prestige. This led them to want to challenge the current world order as they stood to benefit as seizing territories in China and the colonies of the Western powers would both bolster their own economy due to the resources they had but also their own prestige as an imperial power. The British and Americans, on the other hand, had no interest in seeing a change to the status quo as they benefitted the most from it and thus wanted to check Japanese ambitions. This led to the Japanese moving from an ally in WWI to an enemy in WWII for the Entente/Allied Powers

Sources:

• The Defence and Fall of Singapore by Brian P. Farrell

• The Road to War by Richard Overy