In August 1914 he travelled to France as a mechanic with 3 Squadron after war was declared, which followed the German invasion of Belgium. It operated as a reconnaissance unit and McCudden began to fly as an observer. After stopping at Amiens for several days, the unit began reconnoitering enemy positions. 3 Squadron offered support to the British Army at the Battle of Mons in Belgium. That month McCudden saw his first German aircraft on 22 August. On 25 August the British began their retreat, south-west, toward Paris. 3 Squadron moved to no fewer than nine different landing grounds, often delaying departure until the enemy was only a mile or two behind. Eventually they settled at Melun, south of Paris. In the autumn, McCudden participated in locating German artillery positions as the Allied armies drove back the enemy at the First Battle of the Marne and First Battle of the Aisne. McCudden flew these missions with a rifle since aircraft lacked any fixed armament.