On 23 August 1939, in response to the diplomatic crisis emerging over the Invasion of Poland, all French Air Force units were mobilised as part of preparations to be ready for imminent combat operations. Various M.S.406-equipped units were deployed along the border with Germany stretching between Luxembourg and Switzerland, intended to support the sizable ground elements of the French Army from the air. During the Phoney War opening phase of the Second World War, a time of relatively low combat intensity, the type's activities focused upon air defence operations with the aim of countering the prolific aerial reconnaissance and probing activities of small groups of Axis-aligned fighters coming over the border, in addition to escorting friendly reconnaissance aircraft. Throughout the Phoney War, a total of 10,119 fighter missions were reportedly flown over the Army Zones on the border, around half of which being flown by M.S.406 fighters.