I’ve been doing a lot of work lately in countries who had major civil wars and internal conflicts. In many of the cases, the wars are protracted decades long affairs; no one decisively wins or loses, sometimes indefinitely. Why didn’t this happen in the US? Was the war as short as it seems comparatively speaking? I suspect the west expansion and wars with Indians had something to do with it.
Similarly, civil wars sometimes make the nation vulnerable to outside forces. Why didn’t any other nation “kick over our sandcastle” during or shortly after the conflict ended? Is this because Europe was largely occupied with internal affairs and Mexico wasn’t strong/established enough?
I'll address the military side of your question first, as the sociopolitical aspect of what you're asking is quite complex.
The Confederacy by March 1865 was like a walking corpse, a skeletal waif that was alive only insofar as it was still able to walk upright (and just barely at that). It's largest armies were either scattered (Early), bottled up (Lee), or unable to mount an effective campaign against far superior forces (Johnston). There was no longer any infrastructure in place to move men, materiel, ammunition, rations, medical supplies, or any of the other critical necessities for persecuting the war.
Although the conflict didn't officially end with Lee's surrender at Appomattox (April 1865), it was the first of several dominos that fell in the next month-plus that effectively put the nail in the rebellion's coffin. Lincoln's assassination proved that public opinion, north and south, had turned resolutely against any kind of violent guerilla campaign that would continue the conflict in the political arena, and the surrender of Gen. Johnston's army by the end of April ended any question of the Confederacy's ability to continue fighting on the battlefields.
Put simply, the Confederacy as both a political body and nation-state collapsed, and by May 1865, with the capture of Jefferson Davis, there simply was no more war to fight (at least in the traditional sense).
Now, many have argued quite convincingly that the "south" of the Reconstruction era moved quickly to pass laws on the local level to reestablish the sociopolitical hierarchy of the antebellum period, and that political violence, local intimidation of African-American communities, the birth of the Ku Klux Klan, and Jim Crow laws were a continuation of the "fight" that characterized the American Civil War. That's a whole other topic of discussion, but it is indeed a valid hypothesis that gets to the heart of OP's main question "Why didn't the US Civil War continue?" Some would argue that it did via these post-war sociopolitical developments. However, in the strictest reading of things, the US Civil War did not continue because the South was soundly, roundly, and unquestionably defeated. What's more, the passage of the 13th amendment in 1865 outlawed slavery, and made moot the impetus of the war itself.
As for why another country didn't come in and take advantage of the situation, this supposes that the Union was somehow "vulnerable to outside forces," which it decidedly WAS NOT in 1865. By mid-1865, the United States had a standing army well over half a million strong, well-trained and tooled up both at sea and on land. It was no more vulnerable after the war than the United States or Soviet Union were at the end of World War II (indeed, quite the opposite: they had their toys out and a surplus of battle-hardened soldiers with recent experience using them).
[Sources: Jay Winik, 'April, 1865'; Doris Kearns Goodwin, 'Team of Rivals'; James McPherson, 'Battle Cry of Freedom']
Hey there,
Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.
If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!