Plenty of Germans were happy to sign up for Round 2 of a conflict that killed millions the first time. There are plenty of rival nations that have fought stupid bloody wars over the same bit of useless land 8 million times.
Making it even worse is that this is likely going to be
seriously personal for the United States. Something that hasn't been mentioned in here are things like Andersonville Prison - a prisoner of war camp run by the Confederacy, whose commander was hanged after the war in our timeline because the place was basically a death camp. In this timeline, Henry Wirz is still alive, and any peace would see an exchange of prisoners, who are going to be heading back to the Union with tales of this horror show. The Wikipedia page is rather detailed, so I can just quote that instead of writing it up myself:
As we entered the place, a spectacle met our eyes that almost froze our blood with horror, and made our hearts fail within us. Before us were forms that had once been active and erect;—stalwart men, now nothing but mere walking skeletons, covered with filth and vermin. Many of our men, in the heat and intensity of their feeling, exclaimed with earnestness. "Can this be hell?" "God protect us!" and all thought that he alone could bring them out alive from so terrible a place. In the center of the whole was a swamp, occupying about three or four acres of the narrowed limits, and a part of this marshy place had been used by the prisoners as a sink, and excrement covered the ground, the scent arising from which was suffocating. The ground allotted to our ninety was near the edge of this plague-spot, and how we were to live through the warm summer weather in the midst of such fearful surroundings, was more than we cared to think of just then.
Unlike the captives, the guards did not become severely emaciated or suffer from scurvy as a consequence of vitamin C deficiency due to a lack of fresh fruits and vegetables in their diet. The poor diets and resulting scurvy was likely a major cause of the camp's high mortality rate, as well as dysentery and typhoid fever. These resulted from filthy living conditions and poor sanitation. The only source of drinking water was a creek that also served as the camp's latrine. It was filled at all times with fecal matter from thousands of sick and dying men. Even when sufficient quantities of supplies were available, they were of poor quality and inadequately prepared.
There were no new outfits given to prisoners, whose own clothing was often falling to pieces. In some cases, garments were taken from the dead. John McElroy, a prisoner at Andersonville, recalled "Before one was fairly cold his clothes would be appropriated and divided, and I have seen many sharp fights between contesting claimants".
Although the prison was surrounded by forest, very little wood was allowed to the prisoners for warmth or cooking. This, along with the lack of utensils, made it almost impossible for the prisoners to cook the meager food rations they received, which consisted of poorly milled cornflour. During the summer of 1864, Union prisoners suffered greatly from hunger, exposure and disease. Within seven months, about a third had died from dysentery and scurvy; they were buried in mass graves, the standard practice for Confederate prison authorities at Andersonville.
During the war, 45,000 prisoners were received at Andersonville prison; of these nearly 13,000 died. The nature and causes of the deaths are a source of controversy among historians. Some contend that the deaths resulted from Confederate policy and were war crimes against Union prisoners, while others state that they resulted from disease promoted by severe overcrowding; the widespread food shortage in the Confederate States; the prison officials' incompetence; and the breakdown of the prisoner exchange system, caused by the Confederacy's refusal to include black Union troops in the exchanges. The stockade became severely overcrowded
A young Union prisoner, Dorence Atwater, was chosen to record the names and numbers of the dead at Andersonville, for use by the Confederacy and the federal government after the war ended. He believed, correctly, the federal government would never see the list. Therefore, he sat next to Henry Wirz, who was in charge of the prison pen, and secretly kept his own list among other papers. When Atwater was released, he put the list in his bag and took it through the lines without being caught. It was published by the New York Tribune when Horace Greeley, the paper's owner, learned the federal government had refused the list and given Atwater much grief. Atwater believed that the commanding officer Wirz had been trying to ensure that Union prisoners would be rendered unfit to fight if they survived the prison.
Prisoners caught trying to escape were denied rations, chain ganged, or killed. Playing dead was another method of escape. The death rate of the camp being around a hundred per day made disposing of bodies a relaxed procedure by the guards.
When Union troops come back home, they're going to be bringing back stories of conditions like those - these men are brothers, husbands, sons, uncles, fathers, and news of their horrific mistreatment in the South during the war is only going to stoke up genuine hatred...and since the Confederacy got out of the war, so too did the men responsible. It isn't hard at all to see this used as a battle cry in the remainder of the Union, and it presents easy ammunition for those who are against reconciliation with the Confederacy. It'll be a festering wound, slowly rotting its way along in the background - this happened, it happened to us, and the people responsible for it are free and honored in their country. Combine it with things like the
Great Hanging at Gainesville,
the Lawrence Massacre, and other incidents throughout the war (and you'd have the same on the other side of the coin - the Confederates aren't going to be happy about some of the stuff the Union got up to, either), and the aftermath of this American divorce is going to be far, far from amicable. On its own it might not be enough to restart hostilities (unless the Union is really sporting for a second match, in which case this can go down like
Jenkin's ear), but it'll definitely shape the post war narrative of what had happened and what needs to be done next. Civil wars are horrific affairs that can bring a
lot of bad blood, tearing whole families apart as one brother goes for one side and the other to the opposite, and that animosity is now written on a national scale. They can be deeply personal wars, and combined with things like Andersonville, will stoke sufficient anger and fury on the homefront as to put the CSA and the US on a collision course. There'll be those that want peace and to avoid another war, sure, but there'll be those who want to avenge their fallen brothers, their national honor, the loss of a war that the Union on paper should've been able to win (something unexplored in this thread - how did the Union lose? That'll have massive implications for both the Confederates and the Union as to what happens next. A Union version of the "stab in the back" mythos could be born, and only serve as fuel for a second round), and they'll meet those who want peace in debate.
And you already know what they'll say.
"Remember Fort Sumter! Remember Andersonville, Lawrence, Gainesville!"
That kind of fury feels like it could make round two not just inevitable, but a very bloody, very bitter affair. You'd need either a war fought ot absolute exhaustion (in that case, how do you get there?), a very successful peace movement (meaning a lot of mothers and wives going "when will it end" rather than being furious their loved ones just got gunned down in the civil war), or the result is just going to be a second war some years after the first, fought with the fury that can only come from a serious grudge.
On the plus side, it means that Dr. Arliss Loveless can build his giant spider machines in the comfort of the Confederacy...
...and we might get to have the coolest of all time lines: a steampunk civil war rematch. This part might be less serious than the rest of the post above, but c'mon, it'd be fun for ASB at least