BERLIN — Traffic on the Rhine halted, trams and cars could not cross a main highway bridge, and about 20,000 Cologne residents on either side of the river were forced to leave their homes and gather, for safety’s sake, in the gyms of schools closed for the day.

Seven decades after the end of World War II, the threat of aerial bombs still disrupts life in Germany with surprising regularity.

A bomb found along the riverbank during excavations for a pipeline forced the evacuation of everyone within about a half-mile radius on Wednesday. That included people in homes, businesses, a nursing home, a 45-story apartment building and a youth hostel.

In the end, it took half an hour to defuse the 2,200-pound bomb, believed to have been dropped by the Americans in the waning years of the war.