After Britain won the Seven Years' War and gained land in North America, it issued the Royal Proclamation of 1763, which prohibited American colonists from settling west of Appalachia. The Treaty of Paris, which marked the end of the French and Indian War, granted Britain a great deal of valuable North American land.
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Brown v. Board of Education was a landmark case in which it was finally proven that segregation in institutions of education was highly unconstitutional, so you could say that that is the greatest effect since it made other schools become open for everyone as well.