Treason

From Findlaw.com:

The fact that treason is the only crime explicitly defined in the U.S. Constitution speaks volumes about just how vulnerable to attack the constitutional Framers considered the new nation. The relevant section on treason (Article II, Section 3) states the following:

Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court.

One of the reasons for adding the crime of treason to the Constitution was to limit the ability of Congress to define or modify it for political purposes (although the Constitution states that "Congress shall have the power to declare the punishment" for treason). The Framers were well aware that in Britain treason charges often were levied against political enemies in an effort to silence opposing viewpoints, so they wanted to make sure it wasn't similarly abused in the U.S.

The United States Code (18 U.S.C. § 2381) repeats the constitutional definition of treason and outlines the range of punishments upon conviction. Specifically, the statute states that anyone found guilty of treason:

...shall suffer death, or shall be imprisoned not less than five years and fined under this title but not less than $10,000; and shall be incapable of holding any office under the United States.
...

Treason charges have been filed in the U.S. only about 30 times, resulting in roughly a dozen convictions. Below are some examples of treason cases:

• The Whiskey Rebellion (1794) -- Several farmers led an armed rebellion against the newly imposed whiskey tax. Two men were convicted, but later pardoned by President George Washington.
• Aaron Burr (1807) -- The third U.S. vice-president, famous for killing first Treasury Secretary Alexander Hamilton in a duel, conspired to invade Mexico and form an empire that would include parts of the United States. Burr was acquitted because there was no "overt act" beyond the conspiracy to do so.
• Thomas W. Dorr (1844) -- Dorr led a rebellion against the state of Rhode Island, protesting against the state's lack of a bill of rights. He was elected governor through a dubious process (resulting in two administrations for about a year), but was convicted of treason by the U.S. Supreme Court and sentenced to life in prison.
• Mildred "Axis Sally" Gillars (1949) -- The American broadcaster was hired by the German Nazi government to broadcast propaganda during World War II. She was captured in Germany, found guilty of treason, and sentenced to 10 to 30 years in prison in 1949, serving 11 years before her release.