The Winter Line proved a major obstacle to the Allies at the end of 1943, halting the Fifth Army's advance on the western side of Italy. Although the Gustav Line was penetrated on the Eighth Army's Adriatic front, and Ortona was liberated with heavy casualties to Canadian troops, the blizzards, drifting snow and zero visibility at the end of December caused the advance to grind to a halt. The Allies' focus then turned to the western front, where an attack through the Liri valley was considered to have the best chance of a breakthrough towards the Italian capital. Landings behind the line at Anzio during Operation Shingle, advocated by the British Prime Minister, Winston Churchill, were intended to destabilise the German Gustav line defences, but the early thrust inland to cut off the German defences did not occur because of disagreements that the American commander, Major General John P. Lucas, had with the battle plan, and his insistence that his forces were not large enough to accomplish their mission. Lucas entrenched his forces, during which time Field Marshal Kesselring assembled sufficient forces to form a ring around the beachhead. After a month of hard fighting, Lucas was replaced by Major General Lucian Truscott, who eventually broke out in May.