The 71-year-old rejects any suggestion that he was a hero that night. "The heroes were the other side," he told Mitteldeutsche Zeitung newspaper, referring to the civilian masses. "I was so on the safe side."
But if Jager had not given the order for the wall to be opened, the night could have ended in bloodshed.
The stand-off began after the East German regime, reeling from weeks of protests and a flood of refugees fleeing to the West via Czechoslovakia and Hungary, decided to open the borders and allow East Germans to travel freely.
But the flustered apparatchik charged with announcing the decision mistakenly said it would take effect immediately, and tens of thousands of East Germans began streaming to the wall, demanding to be allowed across.
"When I saw him say that on TV, I thought, 'What a load of total c**p'," Jager told Reuters. "He should have known East Germans would head straight for the exits when they heard that. But they didn't inform us at all. We were kept in the dark."
As the crowds massed outside the Bornholmer Strasse border crossing he commanded, Jager made a series of desperate calls to his superiors, asking how to proceed. One told him to send the East Germans home unless they had official travel authorisation to cross the wall. But outside, Jager could see that the crowd was growing angry.
When he called again and told his superiors he had to do something, he was told to let a few of the most agitated people in the crowd through, in the hope it would calm the others. But this just made the crowd more restive. Jager began to fear a stampede.
"There were fears they could get their hands on our weapons," he said.
"My border guards were urging me to do something but they didn't know what. They knew their necks would be on the line, so they wanted me to make the decision."
At 11.30pm, he let everybody through. "When I saw the masses of East German citizens there, I knew they were in the right," he said.
"I was only a lieutenant-colonel and didn't have the authority. But when no one from above would give any orders, I was practically forced to take action."
When he told his sister the next day he was the one who had opened the Wall, she said, "You did well".
"It wasn't me who opened the wall. It was the East German citizens who gathered that evening," he told The Local website. "The only thing I can be credited with is that it happened without any blood being spilled."