The First Anglo-Sikh War broke out soon afterwards and Mouton became the only European officer to remain with the Khalsa throughout. With the Spanish engineer Hurbon he planned the entrenchments used by the Khalsa during the Battle of Ferozeshah. These were described by the British variously as excellent or atrocious, though the British general Hugh Gough called them "well constructed" and noticed his artillery had little effect upon them. Mouton advised Tej Singh to attack the British prior to the Battle of Sobraon, while they were awaiting reinforcements, but was ignored. He assisted with the planning and construction of the defences used during that battle, which he allegedly assured Tej Singh were impregnable. During the battle, the last of the war, the British suffered 1,100 casualties in their assault but were victorious. Mouton escaped to Lahore where he was captured by the British and deported to France in 1846.