You could also make the point that both sides were spoiling for a fight, and that is no doubt true. Also, I don't know enough about the history of slavery world wide to be able to judge the credibility of the above historian's theory. I submit it for your review and comment.
Lincoln gave an interesting speech about the war.
One side would go to war to escape the union and the other side would go to war to preserve it, so the war came.
I heard an economic historian the other day who posited that the Civil War was unnecessary. Other countries had ended slavery without going to war with themselves over it. The US was the exception to that rule. In his view, Lincoln ultimately failed, as he allowed the dispute to drift into war.
This is very true. Lincoln is often touted as a great president, when in fact, he was far from it. the government should have paid to send them back to their homeland, which is where most of them wanted to be.
Lincoln himself never claimed that the US would seek to put down the rebellion (i.e. southern secession) over slavery. The debate prior to the war and even years after the war started was over the expansion of slavery to new territories. Lincoln and the GOP (and the Whigs before them) opposed this expansion, while the Democratic party favored allowing the people in the states or territory to decide whether slavery was allowed.
The Emancipation Proclamation was made more than halfway through the war to demoralize the Confederacy and to rally the abolition movement behind an unpopular war effort.
As you mention, other countries ended slavery without a civil war. For example, the UK government simply bought the slaves from the slave owners and freed the slaves, then abolished slavery.
But ending slavery was not Lincoln’s goal. The war was fought over Union; i.e., over whether a state has the right to secede if it is the will of the people in that state to do so.
I do not know enough of antebellum history to say whether the war was unnecessary, but I do know that Lincoln’s abrogations of basic civil rights (such as habeas corpus) were extreme and unnecessary. Many of his political opponents were jailed for simply speaking out against the war.