When the South secedes, they form an alliance with the Imperialist forces in the Mexican civil war. When the South returns, Johnson decides against intervening in the war, letting Confederate companies & people give arms to their former allies. When the Radical Republicans take power, they finally march into Mexico. This begins nearly a century of charismatic Mexican leaders speaking out against the Americans, going to war, sometimes allying with the Germans, losing & granting independence to more areas. The new governments' weakness allows powerful drug cartels to take power & wage war over the corrupt Mexican government. This TL balkanises mexico without changing that much else. The World Wars might end a little later, but the result is the same.