Another what if contigency just popped into my head
Fenian raids in Canada post civil war are more deadly due to British reaction in Ireland due to the assassination
British reaction to the Fenian raids is then also cranked up
Britain blames US for aiding terrorists and having Canadien conquest dreams
US counters with the St. Alban Raid
"They Killed our beloved Queen!" one side screams waving its fist
"What exactly was John Wilkes Booth doing in Montreal in 1864?" the other side retorts with anger
Tempers flare and reason loses out to national egos
Anglo-American war 1866?
What do Napoleon III and Maxmilian do?
Resurgent Confederacy?
Or the Fenians conduct the campaign in the 1860's...
Spring 1862
Queen Victoria in murdered by a Fenian bomb.
Summer 1862
British investigations uncover a New York trail for the bomb (easy, NY was the largest irish diaspora colony in the US).
Autumn 1862
Diplomatic tensions escalate, the US receives a request for extradition of irish citizens which is almost an ultimatum. Lincoln would cave in, Seward leaks the thing to the press. The northern population shifts into millions-for-defence-not-a-cent-for-tribute mode and Lincoln has to show the british ambassador his (Lincoln's, not the ambassador's
) middle finger. Declaration of war ensues (and nobody in Britain gives a shit about the confederate being slave owners). France follows the trend. Union (plus maybe Russia) vs. Confederacy, British and French Empires. The dogs of war howl.