Well Long Island was an example of the entire army fleeing. Washington escaped certain destruction by making camp and setting campfires fooling the British into thinking he was staying when in reality he fled to safety. So it was a retreat, but not deserting necessarily.
Cowpens was a victory for the colonials, although one which resulted in the Colonials getting chased north into NC. Daniel Morgan placed his troops in front of where two rivers came together, which did not allow the Colonials to retreat. Morgan defeated the infamous Banastre Tarleton ( bad guy from Mel Gipson's movie the Patriot who had a record of killing colonials after surrendering, and attrocities against civilians)..and destroyed a significant part of Cornwallis's army..
Cornwallis wanted to defeat the colonials after cowpens and put the main body of his forces in SC into the feild to destroy Nathanial Green's army. Colonials fleed into North Carolina denying Cornwallis the battle he so wanted. The frustrated Cornwallis burned all his provisions in order to allow his troops to match the colonials maneuverability and give chase. Green outran the British pursuit, and when Cornwallis stopped giving chase, Green turned and pursued Cornwallis's now exhausted troops with no provisions... finally cornering Cornwallis's force between Washington and the French Fleet at Yorktown ending the war. Green was a stud.
Trenton was a huge victory for Washington and Green. So not a lot of American defections / dessertions at Trenton.
So I would go with Bunker Hill, which occurred early in the war before the American's really even got organized. It wasn't cunducted by Washington's regulars, but by normal citizens taking up arms before Washington even arrived in Boston. Didn't actually occur on Bunker Hill btw, but the hill next to Bunkers Hill.... It resulted in high British casualties as the British charged up the hill to displace the patriots.. The patriots though eventually fled in response to the heroic, some would say foolish British charge. The British won the day, but also took more than 2.5 times the casualties as the Americans who fled.
I guess you could say the American's deserted too because as I said they weren't actually a army yet.
they were just folks who were pissed at the British in that point trying to force the occupation out of Boston. Which wouldn't occur until Washington erected cannons over the Boston harbor and the British were forced to leave or be destroyed some months latter.
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