GHQ
After the U.S. invasion of Japan in 1945, Japan was occupied by Supreme Commander for the Allied Powers Douglas !McArthur. Within Japan this position became more commonly known as GHQ (General Headquarters). This arrangement of power continued until self-rule was re-established in 1952 by the Treaty of San Francisco.
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Leaders and Personnel of Imperial Japan
GHQ walked a line between purging the most powerful militarists of Imperial Japan, while also shoring up a powerful-enough proto-government against the USSR.
Monarchy
GHQ backed Emperor Hirohito as a legitimate leader of the Japanese, while also forcing him to renounce godhood.
Military
The Ministry of the Army, Ministry of the Navy, Army General Staff Office, General Headquarters, Army, and Navy were all abolished. Japan would remain constitutionally demilitarized after self-rule returned as well.
The Kenpaitai (憲兵隊) was disbanded and police powers were decentralized.
Bureaucracy
GHQ oversaw a purge of civil servants and bureaucrats. Importantly, note that the targeted individuals were simply barred from public service, and that generally only members of the Home Ministry (内務省, Naimushou) were targeted.
In the power vacuum, leftist grassroots leaders quickly gained influence. GHQ shifted focus in 1949 towards a Red Purge, and even began lifting sanctions from powerful militarists that could counter the popularity of communism.
All purges ended in 1952 with the return to self-rule.
Economy
The Japanese economy was left in deep depression and hyperinflation following the invasion. GHQ prioritized the stabilization of the economy, but quickly began making concessions to major economic forces so as to stave off communist sentiments.
Land Reform
Landowners were forced to sell their land to GHQ, which then redistributed the land to farmers. Tenant farmers were often given ownership of the same plot that they had worked on. As Japanese industry trended towards manufacturing in later years, this small-scale agriculture gave way to sanchan agriculture (三ちゃん農業) (sanchan referring to じいちゃん, ばあちゃん, and かあちゃん).
Labor Reform
A general strike in 1947 was banned, and a Red Purge against communists occurred in 1949.
Economic, Monetary, and Fiscal Policy
See here.