At the beginning of the war, Germany had the most advanced chemical industry in the world, accounting for more than 80% of the world's dye and chemical production. Although the use of poison gas had been banned by the Hague Conventions of 1899 and 1907, Germany turned to this industry for what it hoped would be a decisive weapon to break the deadlock of trench warfare. Chlorine gas was first used on the battlefield in April 1915 at the Second Battle of Ypres in Belgium. The unknown gas appeared to be a simple smoke screen, used to hide attacking soldiers, and Allied troops were ordered to the front trenches to repel the expected attack. The gas had a devastating effect, killing many defenders or when the wind direction changed and blew the gas back, many attackers. Because the gas killed the attackers, depending on the wind, a more reliable way had to be made to transmit the gas. It began being delivered in artillery shells. Later, mustard gas, phosgene and other gases were used. Britain and France soon followed suit with their own gas weapons. The first defenses against gas were makeshift, mainly rags soaked in water or urine. Later, relatively effective gas masks were developed, and these greatly reduced the effectiveness of gas as a weapon. Although it sometimes resulted in brief tactical advantages and probably caused over 1,000,000 casualties, gas seemed to have had no significant effect on the course of the war.