PoD after ARW is piece of cake, but after ACW is a challenge - let me try:
After suffering a bloody defeat at Gettysburg the Unions will collapses to let this war go on for ever. An armistice accepting the CSA is concluded, but the Union falls into lasting political turmoil, bordering to civil war.
On the international scene, not at least in UK, the big issue is the ongoing slavery in the CSA, and as the CSA in 1884, intoxicated by the success in the the civil war invades the north "to restore law and order", the international society is outraged, and many people look to the British Empire "to give a lesson to the Slaveowners!". Almost a year and many massacres after the CSA crossed the border to the Union a British lead European Expeditionary Force is landed in Canada and goes south. All over the former Union the Europeans are greeted as liberators, and soon the CSA seeks peace, agreeing to humiliating terms involving abolishing slavery and an International Occupation Force.
In the North nobody really wants to get back to the turbulent years after the civil war, and a amalgation with Canada is agreed upon, but including wideranging autonomy for the new Dominion called "Dominon of Canada, New Foundland and New England" - in short "the Dom".
In the comming decades the relative peaceful times are utilised to develop industry, commerce and culture, while millions of immigrants enter the new territories. Most from Europe but also many from the stagnating south. Soon the decades of USA are a distant past, seen as a shortlived intermezzo in the unstoppable development of the Anglo-Saxon Empire. By early 20th century the King still resides in London, but most agree that the real heartland of the Empire now is the Dom. After the indecisive Great War in Europe the monarchy fell to communist revolts in mainly the British cities, and the Doms used the chance to get full independence, but also used it to get rid of the old akward name. Many proposals for a new name was discussed. "America" was discarded because it was found impropper to name a great country after an obscure Italian sailor, "New England" reminded too much of the arrogant Englishmen, and "New Foundland" was too cold and wet to give any good associations. But "Canada" always had a good feeling stuck to it, and so that it was. Soon the CSA openly talked about joing Canada, which was effectuated in 1933, and they all lived happily on afterwards...
Regards
Steffen Redbeard