GOP front-runner Donald Trump is well-known for his questionable observations and comments. As his campaign continues to gain momentum, one notable observation concerns the protests at his rallies.

Rallies for the candidate are hotbeds of unrest and violence, including in Arizona, North Carolina and Chicago. The last location was deemed unsafe and cancelled.

During a Las Vegas rally last month, he spoke to the crowd: “I love the old days. You know what they used to do (to) guys like that in a place like this? They would be carried out in a stretcher, folks. True.”

His quote circulated around the Internet, with news stories analyzing clashes at Trump’s rallies and the hostility toward protesters and journalists.

One example stood out with the use of the quote on a famous, Pulitzer Prize-winning photo by John Filo: the photo of Mary Ann Vecchio kneeling over the body of student Jeffrey Miller at Kent State on May 4, 1970.

While Trump may not have advocated for shootings, the juxtaposition brings up a powerful reminder from history and to what extent violence and clashes really can be tolerated until it crosses a line. He has made it clear protesters should be treated unkindly, to say the least, and many supporters have followed suit.

Peaceful protest has a place at every event. But the violence seen in the rallies parallels with past events where peace is met with hostility and horror. What extent is history poised to repeat itself? Have people really learned from events of the past?