From the National Archives:
In 1778 all American military officers were required to swear an oath of allegiance to the United States of America and in that oath to “renounce, refuse, and abjure any allegiance or obedience” to King George III of Great Britain. Among those officers taking the oath of allegiance was Major General Benedict Arnold.
Two years later, Arnold would conspire to surrender West Point to the British. His plot was exposed and Arnold defected to the British where he entered the British Army as a brigadier general.
The oath is dated May 30, 1778.