What issue or development has most affected the United States since the beginning of the 21st century?

the federalist party which advocated strong centralization dominated the united states government till 1800. so 1800 election transformed the power from one party to another party for the first time and was thus a turning point in american history. so jefferson called it "revolution of 1800".

explanation:

the viking voyages took place during the middle ages, which was about this time. 500 to 1350. the viking voyages took place during the middle ages, which was about this time.

vote for officials is the right answer