First things first.  I agree with Glenn Reynolds 100% when he writes:

See, here’s the thing — the fact that Trump is “provocative” doesn’t excuse shutting down his rallies by force. …

… Offensive speech doesn’t justify violence. And  here’s a hint for those throwing stones: “He made me feel bad so I had to burn down his house” is the essence of fascism.

For all the truth in what he writes, Glenn is at risk of eliding the fact that Donald Trump isn’t just holding rallies to express his opinion or even to advance a particular political position.  He’s running, auditioning, for the job of President of the United States — “the leader of the free world,” as we used to say — and this turn of events was entirely foreseeable and within a large enough margin of probability that it should have affected his behavior all along.

1. Trump indulges in rhetoric that seems to condone violence.
2. Some among his supporters engage in violent acts — like the guy who sucker punched a peacefully departing black protester and later talked about killing him and the veteran who pushed a black girl down the aisle and later apologized — and images begin hitting the media of black protesters emerging from rallies covered in blood, even as the Internet is abuzz with the saga of a female journalist who, despite being pro-Trump, appears to have been assaulted by Trump’s campaign manager, Corey Lewandowski — an incident to which Lewandowski and Trump chose to respond by denying the incident and going after the victim.
3. This environment creates both motivation and pretense for events like the violent clash witnessed at a Trump rally last night in Chicago, which the candidate canceled out of security concerns.

Again, the creation of pretense does not mean the astroturf progressive agitators were justified in shutting down the rally, but this is why prominent people, especially those with followings and who purport to be leaders, shouldn’t play coy with hints of violence.

Such developments are critical for us to consider as voters decide whether to elect Donald Trump as president.  For illustration, look no farther than Reynolds’s very next post, which links to an American Interest post about fallout from President Obama’s loose rhetoric in an interview with The Atlantic, during which Obama defended his own terrible record by “dissing major world leaders and close allies.”

As the American Interest post concludes:  “This would be somewhat explicable in the rookie year of a presidency, but it’s very hard to understand in the final year of an Administration.”  We don’t have to blame Donald Trump for the inappropriate actions of his left-wing opposition in order to observe that he would start his own first year as president with a huge deficit in the very same area.

In short, the episode is one more example of how following a disastrous Obama presidency with a Trump presidency would put an exponent on the disaster.