Separation of Powers: Supreme court, President, and Congress
Checks and Balances: The president can veto stuff from congress and congress can override the veto with 2/3 vote. Supreme court can declare things unconstitutional.
Judicial Review: Supreme court can declare things unconstitutional.
Federalism: the theory or advocacy of federal principles for dividing powers between member units and common institutions.
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All of Korea would have been communist and much of what you see today with North Korea would probably have been the state of all Korea had the North won.
It also would have spread communism to a much farther degree and the U.S. would not have South Korea as an ally.
