Valid or Invalid? Strong or Weak? As best as you can determine, which of the following arguments are valid? Invalid? Which are strong? Weak? According to the Encyclopedia Britannica, the first use of poison gas as a weapon in modern warfare occurred on April 22, 1915, when the Germans launched a highly successful chlorine gas attack against the Allied positions at Ypres, Belgium. So, the first use of poison gas as a weapon in modern warfare occurred on April 22, 1915.