Bloody Kansas crisis. Wikipedia describes the scene,
Two days later, on the afternoon of May 22, Brooks confronted Sumner as he sat writing at his desk in the almost empty Senate chamber: "Mr. Sumner, I have read your speech twice over carefully. It is a libel on South Carolina, and Mr. Butler, who is a relative of mine." As Sumner began to stand up, Brooks beat Sumner severely on the head before he could reach his feet, using a thick gutta-percha cane with a gold head. Sumner was knocked down and trapped under the heavy desk (which was bolted to the floor), but Brooks continued to strike Sumner until Sumner ripped the desk from the floor. By this time, Sumner was blinded by his own blood, and he staggered up the aisle and collapsed, lapsing into unconsciousness. Brooks continued to beat the motionless Sumner until his cane broke, at which point he left the chamber. Several other Senators attempted to help Sumner, but were blocked by Keitt who brandished a pistol and shouted, "Let them be!