During the Italian War of 1542–1546, Grey was a commander in the expedition against France in 1544, under John, lord Russell, and assisted in the siege of Montreuil. There seems to have been some jealousy between Grey and the Earl of Surrey. Grey had been appointed chief captain of the army called 'the Crews,' and it was arranged in 1545 that this command should be transferred to Surrey, while Grey was to be appointed lieutenant of Boulogne under Lord Poynings. Upon letters from Guînes, however, the king, Henry VIII, ordered Grey to remain in command of his army, while Surrey was sent to Boulogne. Secretary Paget speaks of the sinister means constantly employed to set these noblemen at variance. Grey finally superseded Surrey as lieutenant of Boulogne in April 1546. During the French campaign Grey distinguished himself greatly, especially by his destruction of the Châtillon fortress, which he razed to the ground. The king took Grey into favour, and promised him rewards and preferment, but the promise failed in consequence of the king's death.