Sumter represents the first time there was a serious battle between forces of the rebellion (Confederates) and the Union (US). Up to that point all the clashes--and there were many of them--had occurred without actual combat. Yes, many were violent in the sense that the rebels forced their way into US property. In some cases, the rebels were ordered by state government to leave afterwards because the state hadn't seceded yet.
In over a month of that going on (mid-November to early January), Buchanan did nothing to counter any of this. The US government did organize a few supply and reinforcement ships to head to forts in Southern states that were still in US hands, but other than that, Buchanan let the rebellion go forward.
If anything, the 'summer of hate' led by BLM and Antifa resembles the Southern insurrection far more closely than the one three-hour riot Trump supporters had.