The articles of Confederation explicitly created a perpetual union, stating that no “alteration at any time hereafter be made in any of them; unless such alteration be agreed to in a Congress of the United States, and be afterwards confirmed by the legislatures of every State”.
That said, the war didn’t start because Lincoln wanted to preserve the Union, though he certainly did.
The war started because the radicals in South Carolina thought they needed war.
Remember, prior to the attack on Fort Sumter, only seven states had seceded. Arkansas, Delaware, Kentucky, Maryland, Missouri, North Carolina, Tennessee, Virginia had not.
Virginia had the largest population, most of the shipping, and the only industry in the South. A seven-state Confederacy would have been a poor backwater, easily dominated by both the US and the European powers.
At the time of the attack on Sumter, there was a debate going on in Congress over what to do. Yes, some were calling for war, others were calling for peaceable recognition. Had Congress recognized the secession, which it was very likely to do, Lincoln would have had little recourse.
Lincoln may have wanted war, but the thing is, so did South Carolina. They thought that only war would convince the other eight slave states to join the secession. They saw themselves with a choice between peace and a seven-state Confederacy or war and a fifteen-state Confederacy. So they rushed into war, trying to preempt any effort in Congress to recognize the secession. (And got an eleven-state Confederacy - and that only for four years. So it goes.)
Billy one note.