In 1989, Liu Xiaobo left a comfortable visiting scholarship at Columbia University to return to his native China and thrust himself into a maelstrom.

Thousands of students had occupied Tiananmen Square in Beijing to demand democratic changes and an end to Communist Party corruption, and without hesitation Mr. Liu, a literary scholar with a dissident’s reputation in China, flew home to join them.

In the square he kept vigil, in solidarity with the young. But he also feared for their lives and ultimately implored them to leave, warning that the Chinese troops who had arrived to quell the disturbance might soon open fire. The soldiers did, massacring hundreds.

For Mr. Liu, a mission of political opposition was born. Risking prison, he became one of the Beijing regime’s most outspoken critics, and in 2010 he was awarded the Nobel Peace Prize.