The exhumation had been requested by the family of a fighter pilot who went missing in Vietnam in 1978 and was recommended by the Pentagon. But because of its precedent-setting nature, the decision was referred to the Defence Secretary.
In a statement, the Pentagon said Mr Cohen had made the decision "after weighing the sanctity of the tomb with the need for the fullest possible accounting. If we can identify the remains now, we have an obligation to try ... The families deserve nothing less."
The body has lain in the Tomb of the Unknowns at Arlington National Cemetery since 1984. The family of Michael Blassie, a fighter pilot lost in action, believed at the time of the burial that the body could be his, but government scientists determined otherwise.
Now, with more sophisticated identification methods available, including DNA testing, the family asked for a new chance to establish whether the remains are his.
The Pentagon says that the remains could belong to any one of nine pilots shot down in the same area as Blassie, and have asked families of all nine to give tissue for testing. Some, however, have declined, saying that they are content for the remains to stay "unknown".
The Tomb of the Unknowns contains the crypts of "unknown soldiers" from the First and Second World Wars, and the Korean War. It is a national shrine and has a guard of honour around the clock.
If the remains are identified as those of Blassie, they would be conveyed to the family for re-burial. There would then be a new controversy: whether a new "unknown soldier" should replace him.Reuse content