Excuse if you've done this before, but WI Petain rather than Nivelle had taken command of the French Army in December 1916. Petain would probably not order the disastrous assault on Chemin des Dames but rather a more limited ofensive in the Reims area. Even considering that this would be at best a limited success, the French army would have been spared the morale busting efects of Nivelle's offensive, and the allies would be in much better shape to counter german offensives while they waited for US numbers to be felt.