According to Green, what was the nullification crisis?

Explanation:

The nullification crisis was a conflict between the U.S. state of South Carolina and the federal government of the United States in 1832–33. ... Calhoun, who opposed the federal imposition of the tariffs of 1828 and 1832 and argued that the U.S. Constitution gave states the right to block the enforcement of a federal law.

explanation:

the addition to geographic boundaries that might have  kept the people of the huang river civilization from traveling to far distance region is that they had learned about the other river valley civilizations but feared contact would bring invasion.

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