The stalemate in trench warfare was overcome in two main ways, one tactical and one strategic.

Strategically, the stalemate was overcome because the Germans were losing confidence.  First, there had been unrest within Germany itself with some calling for an end to the war.  Second, the German offensive that had centered on Amiens had bogged down.  When it failed, and when the Americans took Belleau Wood from the Germans, it became clear that the Germans would not be able to advance any farther into France.  This helped to destroy German morale.

Tactically, a new doctrine had been conceived.  This was a doctrine of combined arms that would use tanks (more than 600), air power (more than 2000 airplanes), artillery and infantry (27 divisions) in a coordinated way.  It called for a massing of tanks such as had never been attempted before.  It forewent the usual artillery barrage that tended to tell the enemy where the attack was coming.  This new style of attack was unleashed at Amiens on August 8, 1918.  The attack succeeded and broke the stalemate of trench warfare.