What was the level of brutality during the American Civil War versus other wars of that scale? Are civil wars less violent and brutal than wars against nations?

I know it's not your intention, but comparing the levels of suffering and destruction is distasteful, at least to me personally. It may be that more people suffered in other wars; that does not mean that the suffering the American people had to endure meant less, or was any less real. That said, the American Civil War is often seen as a war of gentlemen, where the chivalry of the days of old was still preserved. It is true that when compared with other civil wars, it seems shockingly civilized. To make this comparison, I will take three examples that lie within the "long XIXth century": the French Revolution, more notably the War in the Vendée, the Spanish American Wars of Independence, with a focus on Simon Bolivar, and the Taiping Rebellion.

The nomenclature of American Civil War is somewhat contentious among a certain sector of historians. Though there isn't a clear definition of what a civil war is, most people will agree that it is a conflict where people of the same country confront each other militarily. However, most civil wars involve two factions fighting for control of the government of one country. The Confederates had no interest in taking over the US, not that they could anyway. It could be better characterized as a War of Independence, not unlike the American Revolutionary War. If the Civil War label triumphed at the end it was because the Union did, and thus it was established that the CSA was never a legitimate government in the first place, at least legally. Thus, it was not a nation tried to secede from another, but a group of rebels attacking their own countrymen and government. A Civil War.

Yet, this helps explain why unlike other leaders in Civil Wars, Lincoln never had to conduct purges. To be sure, Lincoln did take steps to repress treason. His violations of constitutional rights in Maryland and especially Baltimore is especially infamous. Even as a strong supporter and admirer of Lincoln, I must admit that he stretched the letter of the law so that he could jail secessionists or even possible secessionists without proof or trial, including elected officials. His proclamation of unilateral martial law was ruled unconstitutional by Roger B. Taney, the chief justice in a decision in the Maryland District Court, BUT not in the Supreme Court itself. Still, Lincoln simply ignored Taney's opinion and his issual of writs of habeas corpus. Though the administration insisted that it had the necessary proof to bring a trial, no charges were ever brought forward, and people were released as soon as the situation stabilized.

Lincoln never executed anyone for his allegiance. The farthest he went was simply locking them up, which still caused controversy. He justified it as being a necessary action to fight against treason, as his known adage of "are all the laws, but one, to go unexecuted, and the Government itself go to pieces, lest that one be violated?" Interpreting his duties as President and Commander in chief as giving him the right and obligation of maintaining the government and the constitution against all threats, he often took actions of questionable legality in order to effectively pursue the war.

Compare with the leaders of other Civil Wars, who often take "extraordinary faculties" and use them to justify atrocities. In Latin America, at least, extraordinary faculties were used to justify the suspension of the constitution and with it the civil liberties that it granted. The most common result was executions and purges, as the leaders obtained more power but also had to consolidate it against internal enemies. Buenos Aires was especially infamous for its frequent purges, likewise with the French revolutionaries, and many generals found it necessary to execute other generals lest they grow powerful and thus a threat.

Lincoln never would have done this, but if he tried we can be sure that he would have been quickly impeached. For one, he was not a military leader as it was common in other countries' civil wars; he was a civilian elected official, and his government still maintained a constitutional framework and working institutions. He was no Napoleon, Simon Bolivar or Hong Xiuquan; he was not a popular warlord that came to power by overthrowing a government, but a politician who was elected and wanted and had to work within the democratic system. Though some may say that Lincoln did not need to centralize power around himself because most of Congress and the North was Republican, there was a very strong element of resistance to his government by Democrats, Unionists and even some Republican wings. Whereas in other countries leaders resolved these struggles with often brutal purges, Lincoln had neither the desire nor the capacity to break the constitutional system like that, even if he sometimes bent the laws.

The government of the Confederacy was similarly restrained by a Constitution and a working government that would have not allowed Jefferson Davis to become a dictator. There were many who opposed him, but like Lincoln he simply had not the power to carry out a purge. That's because both combattants had strong democratic institutions and traditions that Latin American nations or China lacked. Without outright going into counterfactual history, it is possible that the Confederacy's later instability could have allowed a military coup to succeed, after all, in the last stages of the war the army was the South's life and force. But that did not happen at the end. It bears mentioning that some people suggested the same idea to Union general George McClellan; in his case it's unlikely that he would have attempted it, and it's unlikely that he would have succeeded.

Contrast this situation with Latin America or China, where no such institutions were created. As a result, the army grew stronger, without having to submit to any authority because it was the authority. Even when civilian governments were created, the army often remained as a political body of its own, and it was easy to overthrow the government as a result. And unbridled power often leads to purges, and a more brutal war...

Horrible acts did take place during the Civil War. The bush war in Missouri and Kansas was especially horrifying, with events such as the massacre of 164 civilians in Lawrence. Confederate troops also became infamous for their tendency to massacre black soldiers because they constituted a direct threat to their system of white supremacy - the appalling Fort Pillow massacre is an example. But these events were relatively contained, and realized by guerrillas and single regiments instead of being an institutionalized police of terror. As much as some people insist on the contrary, Sherman's March to the Sea was not some kind of genocide, and his troops were well behaved to the civilians even as they ruthlessly stole, burned and destroyed property and resources.

Compare this with the Infernal Columns that made many rivers run red in the Vendée during the French Revolution, or with Bolivar's War to the Death. Both were actual political decisions taken with the sole objective of forcing the enemy to submit through terror, unlike Sherman's march which only wanted to destroy the enemy's resources, not the enemy itself. When Union soldiers dealt with partisans they arrested suspects and pillaged farms, quite brutal acts that nonetheless pale with the actions of others, who executed everyone and salted the earth.

Of course, factors are interlinked: Bolivar, the Committee of Public Safety or the leaders of the Taiping Rebellion could order these acts because they had all the power in their hands; Lincoln was no dictator, and an attempt to issue such an abominable order would result in impeachment and even a criminal conviction.

Another factor was that Lincoln refused to take part in deadly tic for tacs like other leaders did. When the Spaniards executed some Patriots in Caracas, Bolivar retaliated by declaring a war to the death and murdering every Spaniard he could get his hands on, which in turn rallied the people to the banners of the brutal warlord Boves, who massacred every Patriot. The massacre of Republicans in the Vendée justified the bloody Republican policies applied later. Lincoln could have conceivably turned to those brutal methods; early in the war, he threatened to hang captured Confederate privateers as pirates, and later to execute captured Confederate soldiers if the Southerners carried out their threat of executing black soldiers. In both cases he ultimately backed out, preventing such bloodshed. Even if in theory Lincoln never deviated from his position that the CSA did not legally exist, he treated them as belligerents and their soldiers as prisoners of war. Bolivar, Robespierre and the Chinese treated rebels as criminals, and as a result the rebels had little incentive to peacefully surrender and instead resisted to the death.

Civil Wars, if anything, are often more brutal and bloody that normal wars are because they often involve deep seated ideological conflicts that allow no compromise, unlike general wars which often were small affairs where you seek military victory, not the complete annihilation of your enemies.

Ultimately, the fact that the CSA was seceding, not overthrowing the government; that Lincoln was a civilian leader still limited by constitutional government; and that he purposely prevented an escalation of violence allowed the Civil War to be less bloody and shorter than its contemporaries. The territories of Gran Colombia, with around 2 million people lost some 300k (15%) in 13 years; China lost 20 to 30 million (more that WW1!) in 14 years. But we should still remember those who died and suffered in the ACW: 650,000 people, or 5%. A lower number, yes, but their suffering is just as valid and horrible.

"It is well that war is so terrible, otherwise we should grow too fond of it" - Robert E. Lee