Hey, like it says in the title, I was curious why the U.S. installed parliamentary governmental systems in Japan after WWII, and in South Korea after the Korean War. The U.S. gov't uses a system with a separately-elected executive and two separate houses of lawmakers, so I was wondering what compelled them to change the model for these other nations (presuming the goal was to install a democratic government that would be more compatible with the U.S. going forward.) Thanks!
I'm not familiar with Korea, but Japan already had a parliamentary system, going back to the Meiji constitution. There was a concerted effort on the part of the occupation government to not make too many aesthetic changes; so instead of replacing the Emperor and the Diet with a more American-style system, they kept both but made the Diet all-powerful and more democratic.
The US did not set up a government in Korea beyond the nominal military control from 1945-1948 (USAMGIK - US Army Military Government in Korea). The Koreans themselves set up their own government. In 1948, elections were supposed to be held across all of Korea. The North boycotted the elections as they knew that they'd probably lose and so the south went ahead without them and set up what became known as the First Republic.
The US really had no interest in Korea going forward. They removed all soldiers from Korea in 1948, Dean Acheson left Korea out of the US Defense Perimeter in a speech in 1950 which (in part) led to the North Koreans and Soviets believing the US would not intervene if they attacked South Korea.
Why did the Koreans set up a parliamentary system on their own?
The Joseon government (monarchy) had become weak and ineffectual. Lots of infighting and corruption during the 1800's led to rebellions, Chinese/Japanese intervention, and finally, Japanese colonization. As /u/anscheisc pointed out, Japan already had a parliamentary (read: WESTERN) style of government. The Koreans realized that they had to change to defeat/remove the Japanese. At the end of World War 1, Woodrow Wilson said that each country should determine for themselves their future. The Koreans went to Paris for the peace talks but, unfortunately, Wilson's beliefs only extended to those colonies of the LOSING countries, not the winning ones and Korea was ignored. Still, the leaders of the Korean government in exile studied western politics as a way to form a government that could stand against future aggression. Syngman Rhee, the first president of the Republic of Korea, spent most of his life in the US, lobbying and begging for help against the Japanese.
At the same time, some of the anti-Japanese leaders had been part of the Tonghak group in the late 1800's, notably Kim Gu. The Tonghak (Eastern Studies vs. Seohak, or Western Studies) group pushed for more orthodox Confucianism and less foreign interference. Why, then, did the western style of government win out over the Tonghak once the Japanese were gone? Simple. They were murdered.
Syngman Rhee was in control by virtue of the US not being up to speed on Korean internal politics. Kim Gu was murdered on the auspices that he was a Soviet agent. Others were arrested, murdered, or, not wanting any part of that, fled to the north. But the result was that there was no one to challenge Rhee and he was able to set up the government that he wanted and run it until it was so corrupt that a student uprising in 1960 (against his election fraud) caused him to resign and leave Korea.
TLDR - the US government did not set up a government in Korea. The Koreans did. They chose a democratic parliamentary system because they thought it would be stronger than other forms of government and because the people in charge wanted that form of government.