The key movements in the battle were the successful opposed crossing of the River Rapido at the mouth of the Liri valley by British XIII Corps, drawing in the German reserves, and the outflanking breakthrough on their left achieved by Free French Forces, including Moroccan Goumiers on May 14. This caused the German Gustav Line to collapse and they fell back on the Hitler Line some 10 miles to their rear. The fighting had drawn all of the German reserves into the Liri valley reducing pressure on the Anzio beachhead, some 60 miles northwest and facilitating Alexander's planned breakout there, known as Operation Buffalo, which took place on May 23, simultaneously with the main assault on the Hitler Line.