The battle at Monmouth Courthouse (June 28, 1778) pitted Washington's revitalized army against Cornwallis' veteran troops, the first large battle since the horrors of Valley Forge. It was a British victory, but barely so... Washington developed a plan that was too complicated for his men and the terrain, but nearly pulled it off anyway. A large part of the blame for his failure lay with Charles Lee, who was incompetent and likely mentally deranged, but Congress always stuck Washington with him. Although a loss for the Continentals, the battle had the very positive affect of exposing Lee's incompetence, and he was never to play a role in the war after that.
Let us suppose that Lee actually does as he is supposed to and Washington wins at Monmouth Courthouse. This would have been a blow to the British, who lost a lot more men than the Continentals did even in winning... they'd have lost many more in losing. But this would also leave Lee in his command of a big part of Washington's troops, giving him every opportunity to commit a major screw up later in the war. Surprisingly, Lee was actually a rival of Washington for command of the whole rebel forces, and it was Monmouth COurthouse that squelched that ambition....