President Truman refused to bomb China because he believed it could lead to a war with the Soviet Union.
The separation of powers was the first step to limiting power for one branch. The US built up their government based on the British, in which they had their version of Parliament, called Congress, the Federal Judge, or supreme court judge (as well as others, but we are focusing on the supreme court), as well as the executive branch, which is the President.
The next step was "checks and balances", which ensured that no one branch can be too powerful. The US president can veto Congress's legislation, Congress can override the veto, they can also impeach the president. The Supreme Court can rule laws unconstitutional, and change the laws based on what they think is constitutional.
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