It was a standoff that could easily have erupted into World War III. In late October, 1961, American tanks stood face to face with Soviet tanks at Checkpoint Charlie in the heart of Berlin, a single wrong move could have been enough to start yet another eruption of continent-wide violence -- or worse.

The Soviets, though, had an advantage. Among the West German police on hand at Friedrichstrasse that day was a man named Karl-Heinz Kurras. But in addition to being a highly respected West Berlin officer, Kurras -- as recent revelations have made clear -- was also a spy for the East German secret police, the Stasi. And he spent the standoff passing valuable information on American positions across the border.

"The headquarters of the Americans is on the first floor of a building on Friedrichstrasse," Kurras reported, according to a story in Thursday's edition of the German tabloid Bild, citing documents from the Stasi archive in Berlin. "A wall of sandbags is in front of the building." He also reported on the US troops manning the tanks and, once the crisis ended, gave information on US tank positions nearby.