The wilmot proviso (1846) tried to forever ban slavery in territories that were gained after winning this war

The correct answer is the Mexican-American War.

The Mexican-American War resulted in the United States gaining a significant amount of territory in what we now know today as the American Southwest. Thanks to the Mexican Cession, the United States gained California, Arizona, New Mexico, Utah, etc.

After gaining all of this territory, the issue then became whether or not slavery would exist in these territories. This is when David Wilmot introduced the Wilmot Provisio, in which he attempts to stop slavery from expanding into these territories.

However, this proviso is never passed. Instead, Congress passes the Compromise of 1850.

the great wall of china worked as a defense for the feudal states of the zhou dynasty to the north. during the warring states period more walls were put up to defend the borders of the different kingdoms. it was semi successful because despite the wall, the mongols eventually conquered china also the great wall provided protection to the economic development and cultural progress, safeguarded the trading routes such as the silk road, and secured transmission of information and transportation. @jayxboof