delay the civil war till 1914.
the South rebels ,not due to slavery but over the rights of blacks to vote or something like that.
You'd see a civil war with barbed wire machine guns, and bolt action rifles.
Trench warfare not unlike the Western Front. Its a slugfest, a meat grinder never seen before.
Mexico intervenes, and allies with the union, in exchange for its former territories back. the opening of another front and fresh troops is to much for the South.
With mexican cores largely untouched by war, Greater Mexico becomes the most powerful country in America.
it has the other effect that the USA does not participate in the Great War, financially or otherwise. so Britain and France dont get that line of credit they otherwise got.