 Documentation regarding German war crimes in World War I was seized and destroyed by Nazi Germany during World War II, after occupying France, along with monuments commemorating their victims.Poison gas was first introduced as a weapon by Imperial Germany, and subsequently used by all major belligerents, in violation of the 1899 Hague Declaration Concerning Asphyxiating Gases and the 1907 Hague Convention on Land Warfare, which explicitly forbade the use of "poison or poisoned weapons" in warfare.In August 1914, as part of the Schlieffen Plan, the German Army invaded and occupied the neutral nation of Belgium without explicit warning, which violated a treaty of 1839 that the German chancellor dismissed as a "scrap of paper" and the 1907 Hague Convention on Opening of Hostilities.