The Great War saga
Smarting from two defeats at the hands of the Confederate States of America, which was allied with the United Kingdom of Great Britain and Ireland and the French Third Republic, the United States has turned to an alliance with the strengthening German Empire. The US military has been reformed along German lines, and the Republican Party of Abraham Lincoln has collapsed, discredited by the defeats, leaving the Democratic Party and the Socialist Party (whose rise was aided by the defection of Lincoln and his supporters from the Republicans to the Socialists) as the main political parties in the United States.
When in 1914 the Great War breaks out in Europe, both the USA and CSA join almost immediately on the side of their respective allies, and the USA is fighting a two-front war against Canada to the north, and the CSA to the south. In eastern America, the conflict soon bogs down into trench warfare, while in the West the battle lines are more fluid.