Following the first chlorine attack by the Germans in May 1915, the British quickly moved to recruit scientists for developing their own gas weapons. Gas research escalated on both sides, with chlorine followed by phosgene, a variety of tear gases, and mustard gas. A wide array of research was conducted on the physiological effects of other gases, such and hydrogen cyanide, arsenic compounds, and a host of complex organic chemicals. The British built from scratch what became an expansive research facility at Porton Down, which remains a significant military research institution into the 21st century. Unlike many earlier military-funded scientific ventures, the research at Porton Down did not stop when the war ended or an immediate goal was achieved. In fact, every effort was made to create an attractive research environment for top scientists, and chemical weapons development continued apace—though in secret—through the interwar years and into World War II. German military-backed gas warfare research did not resume until the Nazi era, following the 1936 discovery of tabun, the first nerve agent, through industrial insecticide research.