"A British Sherman tank and jeep of 4th Armoured Brigade entering the ruins of Cassino, Italy, 18 May 1944." A Sherman tank and jeep of the 4th Brigade entering the ruins of Cassino. The monastery of Cassino had formed the focal point of the German Gustav Line which they had successfully defended since November 1943. The fourth offensive led by Polish and British troops secured Cassino for the Allies and caused the Germans to retreat north of Rome, which was then declared an 'open city'.