None of you are actually answering the OP’s question, even though this is in Great Debates now. He asked “why was secession such a problem that it was worth losing all of those lives?” He didn’t ask who “started the war”, which is a pointless discussion that never goes anywhere.
If Lincoln had not wanted a war, he easily could have ignored the capture of Ft. Sumter. In fact we had ignored and have ignored far greater provocations from other countries over the years without declaring war. It ranks as “relatively mild” in terms of provocations.
Lincoln genuinely believed that a divided United States would ultimately be disastrous for the country. Instead of being a great colossus that spans from the Atlantic to the Pacific, with no serious challenger to the title of master of North America instead we’d be a more fractured continent. While the Northern States would undoubtedly be the stronger, the South would always be a thorn in the North’s side. Instead of being separated by two great Oceans from the world’s troubles and any chance of serious conflict, we’d have a troublesome neighbor beside us at all times. While Lincoln probably didn’t foresee the two World Wars, it’s undoubtedly the truth the CSA’s existence would have gravely complicated America’s position in both wars. Instead what Lincoln probably saw is the politically fractured States of Europe basically fought wars constantly for the prior 1200 years, Lincoln was not formally educated but he adored reading and read books like some men of his time drank whiskey. I think he literally felt that his phrase from the “house divided” speech was true. That a divided country would eventually degenerate into the fractured political reality of Europe and all the constant ruinous warfare that comes with it.
Lincoln also was a lawyer and a student of the constitution. It was his interpretation that his duty as President required him to preserve the Union founded by Washington, Adams, and all the other Founding Fathers. It was his interpretation of the constitution that secession had no legal or constitutional basis. That the Southern states had no right to leave the Union and that he had a responsibility to quell domestic insurrection. Lincoln was a grave and serious man, who took his oath of office seriously.
The South was an aristocratic society with only a veneer of democracy. The South seceded to protect its aristocratic ruling class, or rather, that class caused secession to protect themselves. And protect themselves meant protecting slavery, which they viewed as necessary to maintaining their place at the top of society.
**But Lincoln fought the civil war for legal and moral reasons primarily relating to his beliefs about a unified United States and his legal responsibilities as President. **It was fortunate for Lincoln the Confederates attacked Ft. Sumter, as it gave him the pretext to raise large amounts of troops–and once that happened and the rest of the Southern States seceded it helped commit a shaky North to the war, at least at first.
While the South may or may not have “started” the Civil War, it was not a defensive war where the North had no choice but to invade and fight for four years after Ft. Sumter. There was no imminent risk of invasion from the South, and the South would have been happy to negotiate after Ft. Sumter. Lincoln mostly was not willing to negotiate with the South as long as they remained in a State of rebellion, as he viewed it as giving credit to their claim of being an independent state. So there’s a lot more to why we actually fought the war that killed so many, and it wasn’t because of a silly provocation in South Carolina.
Also to note, while historically the number of dead has long been “fixed” in the public consciousness as 620,000, a pretty respected/accepted analysis of statistics from the era has now revised that figure to around 750,000.