Benedict Arnold's treason wasn't a spur-of-the-moment idea. Married to an alleged loyalist, Peggy Shippen, Arnold carefully planned how he would change sides. It wasn't enough for him to wear a different uniform. He was going to cause serious damage on his way out. The Oath of Loyalty he had signed no longer mattered to him.
An early hero of the Revolution, who owned a mansion (Fairmount Park) in Philadelphia, Arnold began to think Britain would win the war. He didn't want to be on the wrong side if that happened. By 1778, he initiated a treasonous dialogue with the British.
John Andre (in charge of British secret intelligence) wrote a letter on May 10, 1779, to Arnold's go-between (Joseph Stansbury), laying out the terms of Arnold's betrayal. It wasn't just about picking the right side. Arnold's treason was about money - lots of money if Arnold facilitated successful Redcoat attacks against American troops. Referring to Benedict Arnold as "Monk" (a Scottish general who had earlier betrayed the British army), Andre observed:
...that in case any partial but important blow shou'd by his means be Struck or aimed, upon the Strength of just and pointed information & cooperation, rewards equal at least to what Such Service can be estimated at, will be given.
Andre was an Adjutant-General to the British army. It was his job to help the Crown win the war. But Benedict Arnold's actions are hard to understand. Not only did he leak intelligence to the other side, by 1780 he was in a position to cause death to Americans when Washington gave him the command of West Point. A strategically significant American stronghold on the Hudson River, West Point in British hands would have split the colonies in two.
Knowing Arnold had already scattered his troops (to diminish West Point's defenses), General Clinton (who had taken over the British high command from General Howe) made ready to capture West Point. By September 19, 1780, all that remained were final negotiations between Andre and Arnold.