It was assumed by many that in the event of war between France and Prussia, an attack through Belgium by either side might reasonably be the first act, particularly after Emperor Napoleon III's attempt to annex Luxembourg in 1867. Indeed, in the early part of the war, French Marshal Canrobert brought an entire Army Corps (4 infantry divisions) to Châlons-sur-Marne in northern France as a reserve and to guard against any Prussian advance through Belgium. The Belgian army was called up on the 15 July, the same day that both French and German armies mobilised.