The summary would be: Japan's long-term goal was to be in the top rank of world powers. Specifically, in an era of colonialism and economic exploitation, Japan wanted to be on the winning end of those situations. WWII was an extension of this.

Japan went to war to win control of key locations in Asia (mostly from UK and Netherlands), to obtain resources it needed to remain strong and not be subject to disastrous embargoes from European nations and the USA. Having done that by force of arms, it would continue its efforts to exploit China, extend such efforts to other places in Asia, etc. Japan hoped to replace western nations as the beneficiary of Asian trade and resources, and to insure they had a free hand on the mainland of China without being subject to interference by economic pressure from the West.

So, background:

During the mid 19th century, Japanese leadership undertook to remake Japan from an isolationist nation into a "westernized" nation. One powerful motivation was seeing how China had become dominated by European nations. Japan itself was forced to open to European and American trade by military threat. Japanese leaders saw modernization as the only way to avoid the fate of being an exploited outpost for "the west".

Japan undertook government reform and extensive government-sponsored industrial projects. See also Meiji Restoration, Japanese railroad building, etc. Japan entered into more and more active trade and diplomatic relations.

Japan began to eye China with hopes of carving out value for itself, as the European nations had. Japan sent troops to help put down the Boxer Rebellion, and relive the siege of western embassy forces in Peking, but despite this got meager compensation. While some Japanese writers saw China's fate in stark geopolitical factors, others had feelings of cultural and racial superiority. Japan-China attitudes are complex because China was seen as an ancient source of wealth and wisdom; many aspects of Japanese culture, including the writing system, owe a large debt to Chinese influence. But there was also an attitude towards China being sleepy, obstinate, fossilized, and weak. As often is these case these feelings became associated with the people of China, and stereotyped by the Japanese.

Japan began to expand its influence in Korea, fighting a war with Qing in the 1890s. Korea gaind nominal independence from Qing as "Korean Empire", for the most part a vassal of Japan. Japan's ambitions here brought conflict with Russia, which Japan won. Japan's control of Korea had tacit approval from the United States. In 1910 Korea was formally brought into the Japanese empire. There was definite racism by many Japanese towards Koreans.

Although Japan was not bound by treaty to enter WWI, in the first days of that war, Japan made a deal with the UK to enter the war on the Entente side, and secure the moderate number of German colonies and outposts in the Pacific. Many of these were awarded to Japan permanently or in trust as part of the peace process in 1918-1920.

By the interwar years, Japan was the most industrialized nation in Asia. The western nations had established that one road to success was to combine modernization and industry, with the exploitation of weaker nations. Japan basically said, "yeah we wanna do that to!"

As always with any society, there were elements that were against such actions, elements in favor, and many people who just went along with whatever brought prosperity or national pride, and manywho didn't concern themselves with such issues.

Throughout the period leading to WWII, Japan continued to exploit mainland China. As their actions became more obvious and egregious, the Western nations began to express displeasure, and over time diplomatic and even economic sanctions grew. Or, to reframe it, the hypocritical West, having centuries of exploitation and its benefits under their belts, saw Japan as a competitor and wanted to deny Japan her rightful domination of her nature sphere of influence in Asia.

In 1933 the League of Nations condemns Japan's creation of a puppet state in Manchuria. Japan withdraws.

In 1937 Japan gos ham on China, full out open warfare. The USA responds with increasing economic sanctions.

This crisis point in Japan's external expansion was many-fold.

1. China is big, and the more Japan conquered the harder it was to manage.
2. Russian friction wasn't new, but the post-revolution Soviet government continued to consider Japan a competitor. Japanese creation of a puppet state in Manchuria, Japanese determination to deny the USSR a true all-season port on the Pacific, Japanese proximity to the crucial trans-Siberian railroad, all added to tensions.
3. Chinese internal conflict broke out into a full scale civil war, with the communist rebels at least somewhat sponsored by the USSR ... and BOTH sides hated Japan.
4. Crucial modern resources were, by luck of geology, lacking in Japan's possessions. Oil was one major item and directly contributed to the timing of the war.
5. These lacks, besides being a long-term problem for industrial growth, could be crippling if only a few nations denied Japan trade. And sure enough, as noted above, "economic sanctions" applied to Japan were very effective.
6. Domestic politics played a big part. The strongly expansionist nationalist militarist party often held power by a slim margin, and used domestic tools (including assassination and intimidation) and foreign distractions to stay in power. Setbacks in China, casualties, moral concerns, displeasure with the domestic economy and the embargoes ... these were important political issues.

So here we are, rolling into 1939. Now the UK and Netherlands are involved in a major war back in Europe. Despite this, these two and the USA continue to deny Japan access to crucial resources, especially oil. After the fall of France in 1940, Japan occupies French colonies in SE Asia. In 1941 the USA retaliates with a TOTAL FREEZE OF JAPANESE ASSETS in US control, cutting off most of Japan's trade world-wide and nearly 90% of her oil supply.

Famously, Japan looks at their dwindling oil stocks, and realizes that they get WEAKER Each month. In a few years they will be unable to sortie their modern, capable battlefleet at all. Over the course of 1941, there are numerous moments when Japan nearly caves in to the pressure from the West, but in the end the optimistic military clique triumphs, and Japan goes to war in December 1941.

Given all this, the post-war goals seem clear and ... logical if not quite reasonable.