I have seen a lot of discussion about causes of the Civil War. Usually talking about slavery, states rights, and things like that. But almost always, people are just talking about the reasons why the South was willing to go to war. These reasons don't fully explain the North's position.
To simply say that the war was "about slavery" would give one this flowery view that the white people of the North loved blacks so much that they were willing to go to war and die just so that the blacks wouldn't have to be slaves anymore. And I don't buy that for one moment. So what were the North's REAL motivations?
I've heard that the North just wanted to protect the union. But why was that so important to them? Why was it so important that they were willing to send their people to fight?
Could it also have been something similar to the wars that we have today, where the leaders simply tell the people that they are fighting for other people's freedom, but they are actually fighting to give more wealth and power to those in charge?
The single most important reason that the North went to war in 1861 is this: the South started it.
On December 31, 1836, South Carolina passed a law that stated, in part:
Resolved, That this state do cede to the United States, all the right, title and claim of South Carolina to the site of Fort Sumter and the requisite quantity of adjacent territory
Fort Sumter and other fortifications around Charleston Harbor were legally United States territory, regardless of the legality of South Carolina's secession. Anderson and his garrison had an absolute legal right to be there.
At the same time, it was intolerable to the Confederates that the Federal government retain a fort in the middle of one of the South's most important harbors. Confederate agents were sent to Washington to negotiate for the surrender of the fort, but Lincoln refused to meet with them because he did not recognize the Confederacy as a foreign power, and opening negotiations with representatives of the Confederate government would likely be considered a recognition of their legitimacy internationally.
At this point, both sides knew that war was basically inevitable. There was still some hope that there could be a negotiated settlement - Lincoln hoped that the sentiment for secession was not as widespread as it really was, and that some compromise could be reached that would bring the seceded states back into the government, while Davis hoped that Lincoln would back down and consent to the separation. There was essentially zero chance of either of those things happening, so both sides were trying to ensure that when the war started, they wouldn't be the one who started it.
Finally, Lincoln decided to resupply the fort - but only with provisions, not men, arms or ammunition. He notified the governor of South Carolina - his agents making it clear that it was merely a formal notification from the President to the Governor of one of the United States - a courtesy, not a negotiation. This put the ball in the Confederates' court.
They could allow the provisions to arrive, but that would extend the status quo, which was intolerable to the South, indefinitely. Or they could attack the fort, and take the blame for starting the war. They chose the latter. They didn't really have much choice.
For Northerners, this changed the equation. It was no longer a question of whether or not they should go to war to preserve the Union - they were at war now. The question was whether to surrender or accept the war that had already begun. Much like public opinion after Pearl Harbor, there was no longer any question that the country needed to go to war.
Whether or not you 'buy for one moment' that, "the white people of the North loved blacks so much that they were willing to go to war and die just so that blacks wouldn't have to be slaves anymore", there is some evidence that the fight against slavery had a popular emotional appeal to the troops of the U.S. Army.
They may not have 'loved blacks so much', but they did feel that the fight against slavery was a righteous fight.
If the emotions of the troops can be judged by the lyrics of the songs which were most popular with them, then the fight to end slavery was a popular reason for fighting and dying.
John Brown's Body
"Old John Brown’s body lies moldering in the grave,
While weep the sons of bondage whom he ventured all to save;
But tho he lost his life while struggling for the slave,
His soul is marching on."
The Battle Cry of Freedom
"We will welcome to our numbers the loyal, true and brave,
Shouting the battle cry of freedom!
And although they may be poor, not a man shall be a slave,
Shouting the battle cry of freedom!"
The Battle Hymn of the Republic
"In the beauty of the lilies Christ was born across the sea,
With a glory in His bosom that transfigures you and me.
As He died to make men holy, let us die to make men free,
While God is marching on."
Marching Through Georgia
"Hurrah! Hurrah! we bring the jubilee!
Hurrah! Hurrah! the flag that makes you free!
So we sang the chorus from Atlanta to the sea
While we were marching through Georgia."
https://en.wikipedia.org/wiki/Music_of_the_American_Civil_War
So, if we believe that the lyrics of the popular and commonly sung songs of the U.S. Army in the Civil war reflect the emotions of the troops, ending the evil of slavery seems to have been a powerful and popular motivation.
For other, (perhaps more rational than emotional) reasons for the U.S. going to war to prevent the Southern states seceding to protect the slaver society and the institution of slavery (before it had even actually been challenged by any legislation), the FAQs on this sub-reddit will give you a good background.
You might start with:
https://www.reddit.com/r/AskHistorians/comments/svoo6/causes_of_the_american_civil_war/
Which pretty clearly lays out that the 'rational' cause of the U.S. going to war was to present the secession of minority populations caused by fears of a constitutional majority decision. (To prohibit the spread of slavery into new territories.)
If any minority which did not like a possible constitutional majority decision could just decide, "Well, we don't like the way we think this might go. We are out of here." Then, as Lincoln indicated in the Gettysburg address; government of the people, by the people, and for the people, might well perish from this earth, to be replaced by anarchy, dictatorship, or something else.