In a word, on September 18, 1931, the Manchurian (Mukden) Incident marked the dawn of Japanese military aggression in East Asia.The Kwantung Army claimed that Chinese soldiers had planted a bomb on the tracks of the South Manchurian Railway but it was really Japanese soldiers.
Damage to the railway was minimal but the incident soon resulted in the speedy and unauthorized capture of the city of Mukden followed by the occupation of all of Manchuria. The civilian government in Tokyo absolutely opposed this rogue action but they were unable to control the Kwantung Army, and even orders from army headquarters was not always heeded by field commanders.
An architect of the Incident was Colonel, Ishiwara Kanji, who developed his ideas while teaching at the Army Staff College, and was later assigned to the field with the Kwantung Army. Ishiwara possessed an apocalyptic vision of the future, including the coming of a total war, with all the new merging techologies including air power, poison gases, tank batallions, etc. He saw the US as the emerging Pacific Power with whom Japan would have to contend while Japan represented the most advanced, the "leading nation" in Asia. Some kind of Armageddon type of clash loomed as inevitable in his eyes. BTW, he was also a Nichiren Buddhist which also seemed to foretell the coming of a great clash of civilizations way back in medieval times.
In spring 1931, Ishiwara was working with younger officers, convincing them that the Kwantung Army had to take unilateral action and occupy Manchuria even without authorization from the civilian governmetn at home. They started with a plot to elminate warlord Zhang Zuolin who controlled terriotories North of the Great Wall (i.e., Manchuria) so they blew up his train and assassinated him. This was another case of the military acting independently, on its own; but they beleived they were carrying out a sacred mission. His son, Zhang Xueliang succeeded his father, though, and he would not trust nor cooperate with the Japanse.
Next, they set explosives on the South Manchurian Railway (built by Japan) and claimed it was Chinese bandits. They took this as a pretext to enter Mukden and then occupy ALL of Manchuria. Colonel Doihara declared himself head of an mergency committee to govern Mukden, effectively detaching the whole province from Chinese control.
The government at home tried to contain the incident and get the army to promise not to enlarge the theater of conflict...but the reality was the Army did not face any serious sanctions so it was not discouraged from acting autonomously.
They installed the last Manchu Emperor, Henry Puyi, as the monarch--a plan hatched by Colonel, Doihara, known as "the Lawrence of Manchuria (at the end of the war, he was tried by the International Tribunal, accused of crimes against humanity, convicted and hanged). In this new puppet state of Manzhuguo, Japanese "advisers" occupied key positions in the military, police and civilian government so virtually "ran" the new state. That is why Manzhuguo is usually called a "Puppet State." The US and many other nations refused to recognize Manzhuguo as a legitimate state.
The Lytton Commission appointed by the League of Nations had investigated the Manchurian Incident and in 1932 concluded that it was a fabricated incident engineered by the Japanese military to use as an excuse to invade and take over Manchuria. When the membership voted to accept the Lytton Commission's findings in 1933, Japan dramatically withdrew from the League of Nations--beginning a "going-it-alone" odyssey. For Japan, it is usually held that WWII was the Fifteen Year War that Japan began in 1931 with the Manchurian Incident.
Why did Japan choose this route?
Japan wanted the Great Powers to recognize that it had special interests in East Asia -- it was the most industrialized, most advanced and most stable power in the area; it alreadey controlled Taiwan and Korea as colonies, and was concerned about the rise of Communist forces in China. When the Powers would not grant Japan a "Monroe Doctrine" style special interest in East Asia, Japan felt that the Powers would never play fair with Japan and they decided to operate autonomously. For that, they would need to control territory and natural resources in both East and Southeast Asia. They began to see the ABCD Powers -- America, Britain, China and the Dutch (Indonesia, oil, tin, rubber, etc.) -- as having a "stranglehold" on Japan. So, in Japanese eyes, the war was about liberating Asia from western colonial powers.
Military Historian S.C.M. Paine, in his book The Japanese Empire, makes an excellent point about how the two main options available to a country like Japan were to operate as Maritime Power or as a Continental one. Japan initially opted to function as a Maritime Power, to operate within the rules, the laws, the treaties, all the provisions of international and maritime law, that made it possible for countries to cooperate and prosper and to create wealth through international commerce. "The maritime world order," as he puts it, "is a positive sum game....Continental world orders are zero-sum at best and more typically negative sum, given all the fighting over spheres of influence. The motivating goals [for continental powers] are the confiscation of territory and wealth, but the wars entail damage to both, producing a negative sum. The continental paradigm characterized the preindustrial world where land was indeed the source of wealth because agriculture was the primary economic sector.
After the Industrial Revolution, trade, industry, and service became the primary economic sectors, so land was no longer the ultimate source of power, money was. Money bought armies. And money came mainly from industry, commerce, and service." (178-179) Japan, he goes on to point out, was never geographically suited to become a great land power. It was an island country, an archipelago! Like Great Britain, it depended on international trade for resources and prosperity.
I might want to blend this argumment with the way that some historians, like Peter Duus, see the 1920s and 1930s as a contest, or a power-struggle, between what he calls "Multilateralists," largely civilian officials, diplomats, or navy officers who favored the maritime power model, versus the "Unilateralists," often associated with the Army and the orientation toward seeing Japan as a Continental Power with special needs and unique cultural connections to and interests in East Asia. In Duus' view, the "Multilateralist" viewpoint was associated with professionals in the diplomatic Corps, trained at University of Tokyo in Law, and many Naval Officers, who saw Japan as a Western-type of power; they accepted the rules of the "imperialist" game as laid down in various treaties and international organizations because they believed Japan could play by them successfully. They agreed with the US principle that exclusive spheres of influence in China were not a good idea, and they wanted to keep things in China open. They believed in diplomatic negotiations and they were confident in their ability to hold their own. And, yes, they saw Japan largely as a Maritime Power that depended on keeping trade and commerce going, keeping the shipping lanes open, and avoid conflict and war.
But "Unilateralists," tended to define Japan's interests in terms of a "Continental" (v. Maritime) power. They called for a "Positive Foreign Policy," by which they meant an activist, militarily engaged power on the continent. That is why they favored intervention in Chinese affairs with military power when necessary. To them, the "Multilateralist" approach resulted in "weak-kneed" foreign policy. They felt the diplomats would sacrifice National Harmony and well-being for International Harmony. They saw the "Multilateralists" as too close, to friendly with the U.S. and the U.K. Japan had "Special Interests" in China and they were not only material, economic interests. They had some long-standing spiritual obligation to uphold and protect China from the Powers. So, in this view, Japan was "unique," and unlike western powers; so Japanese "Particularism" (Natioanlism, Uniqueness, the Kokutai, the Emperor System, etc.) comes into play here, too. That is why it was justifiable to send troops into China to protect Japanese interstes when the Northern Expedition got underway. And Japan's interest in Manchuria was also special and unassailable. There was no negotiating on this in their view. Ultimately, the "Unilateralists" pushed Japan toward breaking away from international law and treaties, and "going it alone" as a continental power. But this had disastrous consequences.
As Paine would say, "By the time it had finished, Japan had also violated all the rules of the maritime global order: it had walked out of the League of Nations - the primary rule-making body - ignored its treaty obligations, and attacked its trading partners, one of which was a dominant naval power. Once Japan lost its navy - a likely consequence of attacking the dominant naval power - it could not survive in either a continental or a maritime world. Without a navy it could not protect the home islands from blockade or attack and it soon lost its merchant marine, so it could not deliver troops and supplies to the theater or bring resources home. (179) Imperial Japan flourished in its earlier wars when it was cooperative, multilateralist and focused on its role as a maritime power; when it tried to be a unilateralist continental power, it perished. Literally, it went down in flames.
During ghe 1930s, Japan experienced waves of political assassinations, especially of party politicians and zaibatsu leaders, especially after 1931, so that the term “government by assassination” was coined by the foreign press.
This meant that there was political instability and lack of clarity on the domestic political scene.
No doubt, to add to this, the World Depression of 1929 also played a role in creating feelings of anxiety and the conviction that drastic measures were called for. Who should best lead the country?
We also cannot discount ARMY FACTIONALISM: The army was riddled by ideological and factional disputes. For example, FEB. 26, 1936 INCIDENT aka, the Ni-ni-roku(2-26) Jiken of 1936 in which there was a full-blown coup d'etat attempt to put the government in the hands of the military. It was led by Younger Officers who were inspired by 1911 writings of Kita Ikki. The coup failed but the waves of assassinations and coup attempts weakened and discredited governments especially those led by civilian Party leaders that had become the defacto practice since 1912.
There was also heightened rhetoric about Japan’s sacred mission to save Asia from western imperialists, and that Japan was the superior power in Asia; in fact, they claimed racial superiority abd argued for the need for Japan to have an Asian Monroe Doctrine, and then they could establiah the harmony of the 5 Races with Japan occupying the position of leading race.
At the same time,the government was using all the power at its disposal to: stifle dissent, suppress radical ideologies and mobilize civilian population to support war aims. It was also able to control individual Japanese lives from the top down to create an environment in which no one knew who might report someone else to the government, the "Thought Police" (Special higher Police or Tokko) for questionable loyalty;
They manipulated the myth of divine origins and the symbol of the emperor as sacred and inviolable to channel loyalties upward to the symbol of the state and successfully demand sacrifices from the populace. This is why after the War, the Occupation felt they need to make a point of having emperor Hirohito renounce his divinity.