In the spring of 1939, General Headquarters was called upon to defend the country in the event of Nazi aggression. Poznań Army, under the command of Gen. Tadeusz Kutrzeba, was stationed in Wielkopolska and comprised most Polish units garrisoned there during the interwar period, including Poznań, Gniezno, Leszno and Rawicz. The Wielkopolska Cavalry Brigade, under Gen. Roman Abraham, was also mobilised. The army also consisted of National Defence battalions formed in given areas. The leader of the Poznań National Defence Brigade was Col. Stanisław Siuda, a onetime Wielkopolska insurgent from Wolsztyn.

As predicted, Wielkopolska-German fighting was limited to clashes with German border units when war broke out on the morning of 1 September 1939, although there was local fighting around Kępno and Czarnków as well. One Polish Army unit from Leszno even managed a foray into German territory (towards Wschowa). Given the deteriorating situation on other fronts, however, Poznań Army began to withdraw from Wielkopolska on 6 September. They fought bravely alongside Pomorze Army in the Battle of Bzura on 9-17 September and some of them later got through to Warsaw to defend the capital.