The allied armies began to assemble on the borders of France, and Neumann was with the Prussian forces. Napoleon assembled an army and marched into Belgium where he defeated the Prussians at Ligny on 16 June 1815. Neumann took part in the battle, was seriously wounded, and was taken to a hospital in Düsseldorf.
Ang allied armies nagsimula na magtipun-tipon sa hanggahan ng France, at Neumann ay kasama ang Pruso pwersa. Napoleon binuo ng isang hukbo at marched sa Belgium kung saan siya ang toto Prussians sa Ligny sa 16 Hunyo 1815. Neumann kinuha bahagi sa labanan, ay malubhang nasugatan, at kinuha sa isang ospital sa Düsseldorf.