The Civil War was a great impediment to American expansion. Prior to the Civil War, the US was expanding through North America under the guide of "Manifest Destiny." The Civil War forced the country to concentrate more on reconstruction than on expansion, and not until nearly the 20th century did the US have another war of expansion. By that time, most of the European powers had established their spheres of influence and WW1 was looming around the corner. If you could avoid the Civil War, you might have the US continuing to expand right up until just before WW1.
Here's one possible timeline:
1793 -- Eli Whitney fails to invent the cotton gin. Instead, he focuses his efforts more on interchangeable parts and arms manufacture, eventually founding a company called Whitney Arms, in New Haven, Connecticut.
1808 -- South Carolina becomes the last state to abolish slavery, after years of national debate over its unprofitability (due to lack of the cotton gin).
1812-1814: The War of 1812 lasts one less year due to improvement in American arms manufacture. In addition, the US is able to seize Canada.
1846: The US acquires Oregon.
1846-1848: Mexican-American War: US acquires Texas and Mexican territory up to California.
1853: The Gadsen Purchase is a much more ambitious plan and purchases larger territory from Mexico, including territory south of the Gila River and West of the Rio Grande.
1859: Alaska is purchased from Russia.
1898 : Spanish-American War: The US acquires Cuba, Puerto Rico, and the Phillipines.
1903: The Panama Canal Zone is acquired.
1912: Nicaragua is occupied.
1915: Haiti is occupied.
1916-1917: Second Mexican-American War. Pancho Villa antagonizes American territory, triggering a second war with a rump Mexico that results in its entire annexation.
1917-1919: The Zimmerman Telegram is never sent to Mexico, since it the US is already at war with Mexico by this time. The US never gets involved in WW1 and the war ends in 1919 with exhaustion on all sides. There is still a Versailles Peace Treaty, but it is far less imposing on the Central Powers. Germany agrees to establish an independent Poland on its eastern border, as Russia had been defeated in a separate peace. Poland ends up a German puppet state, used as a buffer between Germany and Russia.
1922: The Russian Civil War concludes with the establishment of the USSR.
1922: Italy becomes fascist.
1923-1933: Austro-Hungarian Civil War leads to fragmentation of the empire into Czechia, Slovakia, Bosnia-Herzegovina, Hungary, and Croatia. Czechia and Slovakia become allies of the USSR to maintain their new sovereignty.
