peer reviewed ; Abstract: Since the Treaty of London in 1839, pre-war planning theories in Belgium oscillated between two positions: on one hand secure and defend which border would be endangered, on the other hand constitute in Antwerp a powerful national sanctuary. This concentration of most of the available military means in Antwerp was far more appropriate in the event of a French invasion. But this perspective decreased after the Franco-Prussian war in 1870, while the possibility of a trespassing by the two belligerents became greater and greater. The fortifications of Namur and Liège were voted by the Chambers in 1887 as a deterrent to both France and Germany. Nevertheless, without totally putting aside the defence of the Belgian southwestern border, high-ranking officers seemed to fear more and more a violation of the eastern border. Due to the completion of fortifications on the Meuse in 1892, German columns were supposed to move forward south of the river, through the Belgian Ardennes. On the eve of the Great War, King Albert I would have preferred to concentrate the army on the Meuse but his Chief of Staff selected a central position in order to maintain and secure his line of communication with Antwerp. The lack of a determined and “standardized” doctrine at the highest level of the army led to a “mixed” or compromise plan adopted in a hurry during the first days of mobilization in August 1914.