For the same reason any other attack happens. To gain victory over the enemy.
Gas attack at Hulluch happened on 1916-04-27.
Halabja poison gas attack happened on 1988-03-16.
Sarin gas attack on the Tokyo subway happened on 1995-03-20.
WWII started with Germany's attack on Poland in September 1939.
The gas attack in Syria occurred on April 4, 2017, in the town of Khan Sheikhoun in Idlib province. This attack resulted in significant civilian casualties and prompted international condemnation.
That would be during WW1, Not WW2. But nonetheless, the Germans first used gas at Ypres in April 1915.
on d-day, the allies began a massive surprise attack on Germany forces in France
Poland is where the camps were, Germany built them and ran them.
So both Nazi Germany and Soviet Russia would have time to build of significant military forces to attack and defend/counter-attack. (Respectively)
Well in Great Britan, United States, France, Soviet Union and Canada (the allied forces) had gas drills incase of gas attack by the axis witch was Germany, Italy, and Japan
The Normandy landings were to establish a beachhead in northern France and open a second European front to fight against Germany.
how does tommo survive the gas attack