Naturally, the hostilities had to begin somewhere, but hostilities became inevitable when the various states began to declare their secessions. I say that because I believe that these declarations were sincere. I am convinced that those who made them really did intend to prevent the U.S. Government from continuing to perform its functions in the southern states and they didn't want to invest the time or energy that it might take to oust the U.S. Government legally or peaceably. So they just tried to tell the people of the United States, the other states and the government of the United States to take a hike. Hostilities became inevitable.
The irony is that the path chosen by the southern states actually hastened the end of slavery. And now it's gone. Forever.
Once again, you assume that secession was not 'legal.' Feel free to read the Tenth Amendment, and then quote the constitutional clause that 'delegates or prohibits' secession. In fact, it was the federal government that refused to settle the issue "peaceably:" barring any constitutional prohibition of secession, the federal troops who remained at Fort Sumter were nothing but armed, foreign trespassers - who were eventually evicted without loss of life...
;>)