At the outbreak of World War I in 1914 Beseler was brought out of retirement and was given command of the 3rd Reserve Corps in the German First Army led by Generaloberst Alexander von Kluck. The German army took Brussels on 20 August and the German command considered the Belgian Army defeated. The main force of the German armies marched toward France, leaving the 3rd Reserve Corps behind. Beseler was ordered to take possession of the city of Antwerp on 9 September. The Siege of Antwerp ended on 10 October, when the Antwerp Mayor Jan De Vos, surrendered the city. Beseler followed the Belgian army and was halted in the Battle of the Yser.