From a standard reference:

"Treason, a crime against the state to which allegiance is owed,
consists of attempting to overthrow the government or betraying
it into the hands of enemies intent on overthrowing it. Under
English law high treason was once so broad a term that it was
used until the 19th century to justify punishing all sorts of
persons who were judged as enemies of the monarch.
  
The U.S. Constitution narrowly defines treason and specifically
declares: "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." This constitutional statement was
strictly followed by Chief Justice John Marshall in the 1807
trial of Aaron Burr, who was charged with treason for allegedly
plotting to establish an independent republic in the Louisiana
Territory but who was acquitted because the prosecution could not
prove Burr guilty of "an overt act of levying war."
  
Due to the unusually difficult burden of proof, less than 40
federal prosecutions for treason have occurred, and
only once in its history has the U.S. Supreme Court sustained a
conviction for treason (Haupt v. United States, 1947). Many state
constitutions have treason provisions, but only two persons have
been successfully prosecuted by states: Thomas Dorr and John Brown.
  
Dorr's Rebellion was an uprising in Rhode Island in 1842 to
secure constitutional reform. At that time the state was still
governed under the terms of the 1663 colonial charter, which
restricted voting rights to male property owners. In 1841 a
reform movement began, led by Thomas Wilson Dorr (1805-54). Dorr
called a convention (October 1841) which drafted a constitution
based on universal male suffrage. The state legislature countered
by drafting its own new constitution; but it was rejected in a
referendum, and Dorr's constitution was approved.
  
Although state government ruled Dorr's constitution illegal, his
party nonetheless held elections, and as a result, Dorr
proclaimed himself governor in April 1842. The official
government then declared martial law, and after some armed
clashes Dorr fled the state. Later sentenced (1844) to life
imprisonment, he was released after one year. In the meantime,
Rhode Island legally adopted (1843) a new, liberalized
constitution."

"The American abolitionist John Brown is remembered especially for
his raid on the federal arsenal at Harpers Ferry, Va., in 1859.
Born on May 9, 1800, in Torrington, Conn., he grew up in Ohio.
During most of his adult years Brown wandered from job to job.
Ill fortune, business reverses, and charges of illegal practices
followed him from the 1820s onward. By the 1850s, however, he had
become deeply interested in the slavery question.
  
Brown envisioned emancipation by massive slave insurrection, but
he did not pursue that goal until the 1859 raid on Harpers Ferry.
Before then, he and five of his sons became embroiled in the
struggle between proslavery and antislavery forces for control of
the territorial government in Kansas. By the spring of 1855,
civil strife had broken out in Kansas and Brown had assumed
command of local Free-Soil militia. Within a year, proslavery
forces had sacked the Free-Soil town of Lawrence, an event that
triggered a bloody retaliation by Brown. During the night of May
24, 1856, Brown, four of his sons, and two other followers
invaded the Pottawatomie River country and killed five helpless
settlers, hacking them with sabers. Brown, who was never caught,
took full responsibility for the act.
  
From then on, Brown became even more preoccupied with abolition
by slave insurrection. Observers often remarked on his magnetic
ability to dominate and involve others in his designs. By 1858 he
had persuaded a number of the North's most prominent
abolitionists to finance his insurrectionary projects. After
protracted conspiracy, delay, and diversion, Brown finally chose
Harpers Fery as his point of attack, hoping to establish a base
in the mountains to which slaves and free blacks could flee.
Brown assembled an armed force of 21 men about five miles from
Harpers Ferry, and on Oct. 16, 1859, they seized the town and
occupied the federal arsenal.
  
The town was soon surrounded by local militia, and federal troops
under Robert E. Lee arrived the next day. Ten of Brown's army
died in the ensuing battle, and Brown himself was wounded.
Arrested and charged with treason, Brown conducted himself with
great courage and displayed considerable skill in arousing
Northern sympathy. Many hailed him as a noble martyr, even as
Southern whites expresed deep outrage at his fanaticism. His
hanging, on Dec. 2, 1859, symbolically foreshadowed the violence
of the Civil War, which broke out two years later."
