As Diana Preston documented in her book, A Higher Form of Killing,
Between April 22 and May 30, 1915, Western civilization was shocked. World War I was already appalling in its brutality, but it had until then been fought on the battlefield and by rules long agreed by convention. Suddenly those rules were abandoned when Germany forever altered the way war would be fought. On April 22, at Ypres, German canisters spewed poison gas at French and Canadian soldiers in their trenches; on May 7, the German submarine, U-20, without warning, torpedoed the passenger liner Lusitania, killing 1,198 civilians; and on May 31, a German zeppelin began the first aerial bombardment of London and its inhabitants. Each of these actions violated rules of war carefully agreed to at the Hague Conventions of 1898 and 1907 and were deliberately breached by Germany in an attempt to spread terror and force the Allies to surrender. While that failed, the psychological damage caused by these attacks far outweighed the casualties. The era of weapons of mass destruction had dawned.While the German began these tactics, their opponents quickly retaliated in kind. All deserve blame.