Actress and political activist Jane Fonda said, "It was the most horrible thing I could possibly have done. It was thoughtless." What 1972 event was she aplogizing for?

In the summer of 1972, at the height of the Vietnam War, Jane Fonda traveled to North Vietnam as part of her anti-war activism. During this controversial trip, she was photographed sitting atop a North Vietnamese anti-aircraft gun, a weapon used to shoot down American planes. The image of her smiling and clapping while surrounded by enemy soldiers was widely circulated, causing a firestorm of outrage in the United States, particularly among military members, veterans, and their families.

The photograph was seen by many as a profound betrayal, appearing to celebrate the forces that were killing American troops. It immediately earned her the lasting and derisive nickname "Hanoi Jane." The incident became a defining moment in her life and career, cementing her image in the eyes of her critics as a traitor who gave aid and comfort to the enemy during a time of war.

While Fonda has never apologized for her opposition to the war, she has repeatedly and profusely apologized for that specific photograph. She has described it as a thoughtless and horrible mistake, a "lapse of judgment" that she will regret to her dying day. The quote in the question is one of her many expressions of deep remorse for the pain that single, iconic image caused.