How did military rule affect the expansion of Japan?

The Japanese were concerned by the Northern Expedition of the Kuomintang. They had backed Manchuria's warlord, Zhang Zuolin, however, he became very powerful and attempted to expand into Northern China, which made him a target for Chiang's Northern Expedition. If Chiang Kai Shek was able to defeat Zhang, Japan's special interests in Manchuria could be hit hard.

Therefore, the Japanese Government planned for its army in Manchuria, known as the Kwangtung Army, to disarm Zhang and force him to retreat back to Manchuria before Chiang was able to defeat him. However, some leaders of the Kwangtung decided to take harsher action.

First, they forced the Northern Expedition to halt at Ji'nan.

Then, they assassinated Zhang Zuolin. The assassins believed the effects of the assassination would give them an excuse to conquer Manchuria.

The Emperor instructed Prime Minister Tanaka to enforce discipline within the army. Despite this, the Generals ignored his requests as they believed that punishing those involved would weaken the army's reputation.

In July 1929, Prime Minister Tanaka was forced to resign due to the fact that he was unable to follow through with the wishes of the emperor.

As a result, as early as the summer of 1929, it was clear that the army had enough power to ignore the government and emperor without facing significant consequences.

What led to the invasion of Manchuria on September 18th 1931?

The May 15 Incident

During the May 15 Incident, Prime Minister Inukai Tsuyoshi was assassinated by 11 young naval officers, who were angered by the London Naval Treaty which limited the size of the Imperial Japanese Navy. They were reportedly attempting to complete the mission of the "League of Blood Incident" which aimed at assassinating wealthy businessmen and liberal politicians.

The officers were put on trial for the assassination, but they were heavily supported by the Japanese population, so they were only given light sentences.

He was succeeded by former Prime Minister (from 1921-22) Takahashi Korekiyo as acting Prime Minister.

The Establishment of Military Rule (a Military Government) in Japan

It was clear that the constitutional government of Japan had lost control of its Armed Forces. 

Due to the Government's intense unpopularity, the emperor's advisors decided it was clear that a democratically elected government could no longer provide Japan with the stability that it needed. 

Following their advice, Emperor Hirohito appointed Admiral Makoto Saitō as Prime Minister. As the armed forces assumed control, Japan transitioned into a military dictatorship.

Thought-provoking question→ Did military rule make war inevitable for Japan?

I will think about this in this topic's episode of my podcast.