To begin with, there is a question of definitions: how do we define World War II? By some definitions, Japan was the first of the major powers to go to war, with its invasion of Manchuria all the way back in 1932, or with the outbreak of the wider war in China in 1937. However, let us save that debate for another time. I am going to make an assumption that by your question, you're asking much more: what led to Japan launching attacks on the Allies in Asia in December of 1941? If that's not the basic gist of your question, please do clarify. I've previously answered a similar question, and that post should cover the topic in a bit more detail than I'll be going into here.

If I were to provide a singular answer to this question, then it would be simple: China. The war in China, combined with the Japanese domestic political situation resulted in Japan having no good strategic options available to it at the start of the 1940s. The continued escalation of the war in China was worsening Japan's relationship with the US, and the Japanese occupation of French Indochina--in order to try and cut supply routes into China--had resulted in the US embargoing exports of vital war material to Japan. On the other hand, both the army--and the civillian population--were insistent that an end to the war in China would have to come with serious concsssions in Japan's favor, if the war were to be worth the blood and treasure Japan had invested into it; concessions that the US would be entirely unwilling to give. If the civillian government were simply to try and force the Army to withdraw, there would: a) be serious concern if the Army would even obey, and b) a risk that the Army would launch a coup against the government to forestall such a decision.

Thus Japan was faced with a strategic dilemna: give in to the US and risk a coup by the Army, or continue the war in China and risk deeper American involvement. At the same time as all this, the mindset of the Navy's leadership is rapidly becoming one of "use it or lose it". With the US oil embargo, the Navy had a limited period of time in which it could launch large scale offensive operations. Similarly, the US Two Ocean Navy Act had promised to build the US a fleet second to none. While the IJN had always planned to fight outnumbered by the US, the fleet under construction in the US in 1940 was simply too big for the IJN to have any reasonable hope of defeating. Yet, with many of the European colonial powers under German occupation or threat of invasion, there seemed to be an opportunity in the south: the European colonies in Malaya and the East Indies. Seizing those colonies would secure some of the resources Japan needed to continue its war in China, but would mean war with the US and UK.

It's important to note that most Japanese leadership saw little hope of victory. Their hope was singularly that their rapid strike south, and crippling of the US fleet at the outbreak of the war would present the US with a fait accompli, and the US would choose a negotiated settlement rather than undertaking the cost of a long war across the whole of the Pacific. It was a longshot, but it was still seen as the only potential 'third way' out of the dilemna Japan found itself trapped in.

Hope this serves to answer your question. Please feel free to ask any follow ups.