As Commander in Chief of the United States military, the President can summon any member of the military to his office. For example, President Obama summoned Gen. Stanley McChrystal to a meeting Tuesday. Regular civilians cannot be made to appear before the President, however.

If a member of the military were to refuse a summons like the one Obama issued to McChrystal, they could be fired. The power of a president to enforce such a summons with a member of the military comes from Article 2 of the Constitution, said William G. Howell, professor of American politics at the University of Chicago.

Presidents also have the power to summon most members of the executive branch, who are, in effect, the President's employees, Howell said. They, too, can be fired for refusing to appear. The exceptions to this are members of independent commissions within the executive branch, such as the one announced last month to investigate the BP oil leak and offshore drilling. Presidents do not have direct control over the hiring and firing of these numerous commissions, and so would have no way to enforce a summons.

As for members of Congress, members of the judiciary or civilians, the President does not have the legal authority to enforce their being summoned.

"Technically, he can summon anyone and ask them to appear, but he does not have subpoena powers," Howell said, so if a civilian were to refuse to appear, there would be nothing he could do.

However, Congress and the courts do have such power over civilians.

Any court in the land could summon a person to appear before it. By issuing a summons, at the beginning of a lawsuit, or by issuing a subpoena, which requires a person to appear as a witness, a court has the power to force a person to show up or be arrested, said Eugene R. Fidell, lecturer at Yale University Law School.

And Congress can also issue and enforce subpoenas. But often, political tug-of-wars play a more important role than subpoenas do in appearances before Congress, Howell said. For example, Congress requested but did not subpoena Condoleezza Rice to appear to answer questions about the Iraq War. She went, but negotiations took place over what she would talk about and how long she would be there.

"I suspect that summons happen a lot," Howell said. "If the President wants to know something, they ask a member of the cabinet. It's not always a formal thing; it's the way our government works."

The most infamous presidential summons in history was issued by President Harry Truman to General Douglas MacArthur, Fidell said. The two had vehemently disagreed over the handling of the Korean War.

The meeting took place on Wake Island, in the Pacific, on Oct. 14, 1950, and MacArthur was late for it.

According to the President's hand-written notes from the meeting, "MacArthur was at the airport with his shirt unbuttoned, wearing a greasy ham and eggs cap that had evidently been in use for twenty years."

Perhaps not surprisingly, Truman fired MacArthur the following April.

How Many Presidents Have Been Divorced?Why Are There 9 Supreme Court Justices?Do Third-Party Candidates Ever Win?This article was provided by Life's Little Mysteries, a sister site to LiveScience.