Richard Neustadt, Presidential Power: Chapter 2, "Three Cases of Command"

Sometimes a president can get things done by command. This turns out to be the exception rather than the rule. Presidential commands require five things in order to succeed (to be self-executing, in Neustadt's terminology): personal investment, clarity, publicity, recognized authority.

Three cases: Truman's dismissal of MacArthur in 1951, Truman's seizure of the steel mills in 1952, and Eisenhower's dispatch of federal troops to enforce school desegregation in Little Rock in 1957. In each case, command was actually the outcome of a failure to achieve the desired result by softer means. It was in effect a last resort.

Furthermore, in each case the command did not actually achieve the ultimate policy aim. In firing MacArthur, Truman hoped to avoid a prolongation and extension of the Korean War. But the firing forced the administration to be explicit about its intention not to attempt the conquest of the North, which removed China's incentive to reach a rapid settlement. In the steel dispute, Truman sought to preserve steel production while maintaining price controls, but a strike and price-control breaking settlement were only delayed. In Little Rock, integration of the local schools was not sustained into the next year.

In each case, the real effect of the command was to keep open further policy options that otherwise would have been closed. But the presidents were not able to fully exploit these options anyway.