Senior Pentagon officials said the change was to ensure that the No. 3 in line would be someone with a wider area of expertise and experience, rather than the Army secretary, whose focus is on the ground forces.

Yet even top Rumsfeld aides acknowledged at the time that the decision had dual motivations: It was an official affirmation of their trust in Mr. Cambone’s experience and intellect — and a slap at the Army’s leadership at the time.

Mr. Rumsfeld had a long-running feud with the nation’s oldest armed service. He had already fired one Army secretary, Thomas E. White, and aides said at the time that he remained dissatisfied with the service’s civilian leadership.

President Obama’s executive order, published March 1, re-establishes the Army to its former place.

If the defense secretary, his deputy and the Army secretary were all hors de combat, authority would then pass to the Navy secretary, then to the Air Force secretary, in the historic order of establishment of the services. Next in line after them would be the under secretary for acquisition, technology and logistics; the under secretary for policy; the comptroller, who is the Pentagon’s chief budget officer; and the under secretary for personnel and readiness. Only after all of them would the line lead to the Pentagon’s senior intelligence official.

White House and Defense Department officials said the new executive order was intended to restore Pentagon succession to its traditional pattern and to render a sequence based on the office, not on the personality in that position at any given time. (The succession plan would also take effect should the defense secretary become incapacitated by health problems, but that could be handled in a calm, deliberate manner across the government.)