The WAR was fought because Lincoln wanted to Preserve the Union. He could have left the South go its way
Not really. He had to resist it after they fired on the flag. The people would have insisted and the Congress would have forced it. But then again, the Confederates needed to start a fight and force Lincoln to react or they would have had no chance of getting the Upper South to join them.
Keep in mind that slavery itself was not in jeopardy in 1860. Expansion of slavery was the crux of the issue and expansion was an economic and social imperative for the slave powers. The economics of slavery were in reality a Ponzi-like model where it remained highly profitable only so long as there was a growing demand for slaves. In the 1820s, slavery was only marginally profitable in the upper south and even old Virginia came close to ending it. With the growth of King Cotton along the Gulf coast, even owners of worn-out plantations in Virginia and Maryland were soon making fortunes, not from crops, but by selling slaves down the river --- over 25,000 a year. With high birth rates among slaves, their supply was endless. It was future demand that was the problem they had to deal with and the Cotton Belt was nearing saturation.
Even if somehow Sumter never happened, and the 7 Deep South states had their way, it would not have been long before a war broke out. Agitation in the upper south and border states would have resulted in local insurrections that Federal troops would need to quell. Virginia, North Carolina, Kentucky and Tennessee were the true economic powerhouses and centers of white population of the slave states and without them, the Confederacy could not have long survived. Confederates would also soon have been making territorial claims in the West or into Mexico, looking for more markets for slaves. With the slave population doubling every generation, expansion was vital to keeping the Ponzi scheme alive.
No serious person at the time, north or south, for a moment thought that secession could be accomplished without war. It was simply too thorny and passionate of a problem. The political reality was that Lincoln could have demanded a 50% tariff and a blank check on internal improvements and the south would have agreed to it if he had only backed down on expansion of slavery. But expansion was the only issue on which Lincoln could not and would not compromise.
War was inevitable and had been since the invention of the Cotton Gin.