In the American army, Arnold was known as the captor of Fort Ticonderoga in 1775 and for his bravery in the Battle of Saratoga in 1777. In the latter, he sustained an injury that ended his combat career for a while.
After watching others take credit for his successes and being passed over for promotions and objecting to the alliance with France, Arnold decided to switch sides in 1779. Obtaining command of West Point in July of 1780, Arnold did so in order to be able to present it to the British.
He was exposed when the American forces captured British Major John André who was carrying papers containing the plot.
Arnold managed to escape down the Hudson to the British ship Vulture, just missing George Washington's army. He entered the British army as brigadier general.
Washington was calm when told of the betrayal. He tried several ways to get Arnold back - in order to hang him, that is - including offering the British André for Arnold, which they refused and André was hung. Washington sent men into New York to kidnap Arnold which would have succeeded if Arnold had not changed living arrangements. Washington then set an order that Arnold was to be hung if ever captured.
Arnold died in 1801 in London.
There are several memorials in the United States dedicated to Arnold, including:
-At Saratoga where there is a memorial, but no mention of his name. It instead says: In memory of the most brilliant soldier of the Continental army, who was desperately wounded on this spot, winning for his countrymen the decisive battle of the American Revolution, and for himself the rank of Major General.
-At West Point on plaques commemorating all the generals from the Revolution. One plaque has no name and reads 'Major General. Born 1740'.
In London, on the house that he resided bears a plaque describing Arnold as an 'American Patriot'.