Without making it implausible wank America as much as possible.
My scenario:
A portion of Canada is annexed by the US in a more succesful War of 1812. The Mexican-American War happens earlier and all of Mexico is conquered. Cuba is annexed when Spain sells it to the US. Santo-Domingo and Haiti also falls to the US. When the Civil War starts also earlier the CSA is quickly deafeated. Around 1880 the Third Anglo-American War starts and the rest of Canada, and all British possesions in the Western Hemisphere is conquered. This US allies with Germany. Meanwhile American imperialism continues, Central American republics fall one by one, Greenland and Alaska is purchased, the Spanish-American War results in Phllipines,Guam,Puerto Rico, and African possesions conquered by the US. Ethiopia becomes a colony of the US. Taiwan and Southern China becomes part of the US when it falls into civil war. By 1915 the US is the largest empire territorialy on Earth....