You are confused or poorly informed about the history of the war between the states.
Neither side ever formally declared war on the other.
The secessionist southern states did not declare war on the United States.
The southern states declared their secession from the Union based on their belief that states, which had voluntarily joined the union, had retained the right to separate from the union.
The federal government did not agree that the states had the right to secede from the union and declared the secession to be a rebellion.
The government determined to put the rebellion down by military force rather than resolve their disputes through diplomacy.
But he did not. In fact, he rejected overtures from the south to negotiate.
If Abe Lincoln was the great statesman and president many believe him to be, he would have pursued ways to solve the dispute without the loss of 600,000 killed, 500,000 wounded and the almost complete destruction of the south.
The 600,000 dead was 2% of the entire US population at the time. That would be equivalent to 6,000,000 dead today.
But the war dead and injured are not the full story.
Lincoln approved of U.S. Grant's tactics of loosing the union army on the civilian population of the south to punish the secessionists. Grant openly stated his determination to destroy the south and Lincoln acquiesced. General Sherman was sent out to burn the south to the ground and that is what he did.
The Union army's brutal raping, killing, pillaging, plundering of the southern civilian population, the destruction of the railroads and other infrastructure, and the burning of cities, homes, farms and businesses, even after the south surrendered, is all well documented in the history books.
Not nearly as confused or misinformed as you, FRiend.
Iron Munro: "The secessionist southern states did not declare war on the United States."
Not true.
On May 6, 1861, three weeks after physically starting war at Fort Sumter, the Confederacy formally declared war on the United States.
Here is the text of the Confederacy's declaration of war.
Iron Munro: "The government determined to put the rebellion down by military force rather than resolve their disputes through diplomacy."
Wrong again.
In his inaugural address (March 4, 1861), President Lincoln announced to the Confederacy that they could not have a war unless they themselves started it, and Lincoln kept to his promise.
So no Confederate soldier was killed directly by any Union force, and no Confederate state was "invaded" by any Union army until after the Confederacy started and formally declared war on the United States.
Iron Munro: "In fact, he [Lincoln] rejected overtures from the south to negotiate."
In fact, Lincoln followed the same policy as his Dough-Faced Democrat predecessor, President James Buchanan.
Neither recognized the legitimacy of the Slave Powers' declarations of secession, neither would "negotiate" with Confederate emissaries, but both were determined to provide the Slave Power with no excuse for starting war.
So, of the dozens and dozens of Federal properties illegally seized by Confederate authorities, Buchanan decided to defend only two: Forts Sumter and Pickens.
After his inauguration on March 4, Lincoln simply followed Buchanan's policies, but immediately learned that Sumter must be resupplied or surrendered.
And Lincoln was willing to surrender Sumter, for the right price, and here was Lincoln's price: the Virginia secession convention must promise not to secede.
Lincoln said, "a fort for a state" was a good deal.
Of course, Virginians' would make no such promise, and so Lincoln decided to resupply Fort Sumter, with food.
But the Confederacy had long prepared for war against Sumter, and took Lincoln's announcement as their excuse to start Civil War, on April 12, 1861.
Even then, Lincoln had no other intention than to recover Federal properties unlawfully seized by secessionists.
But on April 23, within days of seizing Sumter, Jefferson Davis offered military aid to Confederate forces in the Union state of Missouri.
On that same date, US Army officers captured in Texas were treated as POWs.
At this point we should note that, once the Confederacy declared war, on May 6, 1861, any Union citizen who gave "aid and comfort" to the Confederacy met the Constitution's definition of "treason", a law which Lincoln was bound to enforce (Article 3, section 3).
So, as I said, the responsibility for Civil War belongs to those who first started and then formally declared it: Slave Power secessionists.
Lincoln's responsibility was to enforce the Constitution, and defeat declared enemies of the United States.
Iron Munro: "The Union army's brutal raping, killing, pillaging, plundering of the southern civilian population, the destruction of the railroads and other infrastructure, and the burning of cities, homes, farms and businesses, even after the south surrendered, is all well documented in the history books."
In fact, there is virtually no documented record of killing or raping civilians by soldiers of either side, and such deaths as did happen were nearly all accidental.
It's also a fact that when Confederate armies invaded Union states & territories, they often left a trail of destruction and pillaging.
Indeed, some seriously argue: the main reason RE Lee lost at Gettysburg was that his chief eyes & ears, Stuart's cavalry, ran off to pillage the Maryland and Pennsylvania countryside for desperately needed supplies, and was nowhere to be found when Lee needed him most.