In 1791, St. Clair commanded the American forces in what was the United States's worst ever defeat by the Native Americans.
St. Clair was born in Thurso, Caithness, Scotland.
In 1757, St. Clair purchased a commission in the British Army, Royal American Regiment, and came to America with Admiral Edward Boscawen's fleet for the French and Indian War.
In 1770, St. Clair became a justice of the court, of quarter sessions and of common pleas, a member of the proprietary council, a justice, recorder, and clerk of the orphans' court, and prothonotary of Bedford and Westmoreland counties.
St. Clair issued an order for the arrest of the officer leading the Virginia troops.
By the mid-1770s, St. Clair considered himself more of an American than a British subject.
He took part in George Washington's crossing of the Delaware River on the night of December 25–26, 1776, before the Battle of Trenton on the morning of December 26. Many biographers credit St. Clair with the strategy that led to Washington's capture of Princeton, New Jersey on January 3, 1777.
St. Clair was promoted to major general in February 1777.
In April 1777, St. Clair was sent to defend Fort Ticonderoga.
St. Clair was forced to retreat at the Siege of Fort Ticonderoga on July 5, 1777.
St. Clair was at Yorktown when Lord Cornwallis surrendered his army.
St. Clair was a member of the Pennsylvania Council of Censors in 1783, and was elected a delegate to the Confederation Congress, serving from November 2, 1785, until November 28, 1787.
On February 2, 1787, the delegates finally gathered into a quorum and elected St. Clair to a one-year term as President of the Continental Congress.
Congress enacted its most important piece of legislation, the Northwest Ordinance, during St. Clair's tenure as president.
during St. Clair's presidency, the Philadelphia Convention was drafting a new United States Constitution, which would abolish the old Congress.
Under the Northwest Ordinance of 1787, which created the Northwest Territory, General St. Clair was appointed governor of what is now Ohio, Indiana, Illinois, Michigan, along with parts of Wisconsin and Minnesota.
In March 1791, St. Clair succeeded Harmar as commander of the United States Army and was commissioned as a major general.
In October 1791 as an advance post for his campaign, Fort Jefferson (Ohio) was built under the direction of General Arthur St. Clair.
More than 600 soldiers and scores of women and children were killed in the battle, which has since borne the name "St. Clair's Defeat", also known as the "Battle of the Wabash", the "Columbia Massacre," or the "Battle of a Thousand Slain".
The wounded were many, including St. Clair and Capt.