Final answer:
The Election of 1800 marked the first peaceful transition of political power between opposing parties in U.S. history.
Step-by-step explanation:
The described event in the question is the Election of 1800.
The Election of 1800 marked the first peaceful transition of political power between opposing parties in U.S. history. It was a significant moment because power was transferred from the Federalists to the Democratic-Republicans. Thomas Jefferson, a Democratic-Republican, won the election and became the third president of the United States, succeeding Federalist John Adams.