What were the effects of the Napoleonic Wars on the United States?
It resulted in the Louisiana Purchase, for one thing...
In the earliest part of the 1800s, America had two main political parties: The Federalists and the Democratic Republicans. The Federalists, founded by Alexander Hamilton and John Adams, favored trade and friendly relations with Great Britain over France. The Democratic Republicans, founded by Thomas Jefferson, favored France, especially revolution-era France (Jefferson was, if not one of the Revolution's architects, certainly an abetter of them during his time in Paris as the American ambassador).
Jefferson's nuanced relationship with Napoleon kept communication channels between them open. When Napoleon needed to raise some money quickly, he considered Jefferson's offer to buy New Orleans for $1 million and countered with an offer to sell the entire Louisiana Territory for $10-15 million (about $250 million in modern dollars), doubling the US's size and giving it an arguable claim to a port on the Pacific coast (Louisiana Territory's western frontier was very hazily defined).
Federalists in Congress asserted that Jefferson had no constitutional authority to make this purchase; Jefferson countered that the Constitution gave him authority to make treaties, this was a treaty, and in 1803, the deal was done.
Nine years later, the US entered into the War of 1812 with Britain. England's primary military effort at the time was its ongoing war with Napoleon, but they had enough ships and Canadian troops to wage both wars simultaneously. The Federalists strongly opposed America's entry into this war, but popular sentiment prevailed and the Federalist party's influence in Congress dwindled; they never had a majority in the House or Senate again after 1800, or a Federalist president after John Adams. They had less than a quarter of the votes in each chamber after 1812.
The Federalist Party dissolved in 1824, and while relations with England were generally good, America avoided military entanglements with England until 1917.