When Jeff Widener looks at the most important photograph of his career, it makes him think about failure. Like most news photographers, Widener is often worried that he will be absent during a critical moment and miss a critical shot. And like many of the most important photographs in history, Widener’s Tank Man almost didn’t happen. “I don’t have it on my wall,” says Widener, “because every time I look at it, it reminds me how close I came to messing it up.”In 1989, Widener was a picture editor for the Associated Press in Southeast Asia. As political turmoil and student protests heated up in Beijing that spring and summer, Widener was dispatched to China to cover the melee. Day after day, he would leave the AP bureau inside the U.S. diplomatic compound in Beijing and ride to Tiananmen Square to shoot pictures. At first, the assignment seemed relatively safe and straightforward. “The square was actually very organized. They had street sweepers. They had sort of a security ring all the...