During the Second World War, Sidney fought in the Battle of France and the Italian Campaign. While serving as a company commander in the 5th Battalion, Grenadier Guards (itself part of 24th Guards Brigade of the British 1st Infantry Division), he led a handful of men in the defence of the Anzio beachhead in February 1944, for which he was awarded the Victoria Cross. Sidney led a successful attack which drove German troops of the 147th Grenadier Regiment out of a gully. Later he led another counter-attack and dashed forward, engaging the Germans with his tommy gun at point-blank range, forcing a withdrawal. When the attack was renewed, Sidney and one guardsman were wounded and another killed, but he would not consent to have his wounds dressed until the Germans had been beaten off and the battalion's position had been consolidated. During this time, although extremely weak from loss of blood, he continued to encourage and inspire his men.