According to John Green, the Cold War taught us the greatest threat to humans were what exactly, and which two countries were
the main countries involved in this Cold War?

The United States and the Soviet Union.

Explanation:

After the end of World War II, the world was divided into two opposing blocs. The Eastern one led by the Soviet Union and the Western bloc led by the United States. It was a war of communism and democracy, a war not fought with weapons. The Cold War meant a period that was full of arms races, technological supermation, and space conquest. Both sides put pressure on many countries to change their political regimes, and the war was characterized by a race to build a nuclear arsenal.

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