During the Polish September Campaign at the beginning of the Second World War, Modlin Fortress was a headquarters of the Modlin Army until it retreated eastwards. From September 13 to September 29, 1939 it was defended by Polish forces under command of general Wiktor Thommée against assaulting German units. During that time, the fighting was closely connected with the strategical situation of the battle of Warsaw.

Among Polish forces defending the fortress was an armoured train 'Śmierć'. Modlin anti-aircraft battery was the one that shot down the most German planes during the campaign. Modlin capitulated on September 29, as one of the last Polish units during the campaign.

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• Modlin fortress as seen from a satellite - green cross marks the southern bridgehead pictured above