World War II
After the ending of WWI, the people in Europe is very scared of WWII because they saw the casualty number going up, and they know, if there was a WWII, the number would only become higher. WWII is started by Hitler invading Poland, then lead to the Axis power fighting the allies. During WWII, fighter planes, bomber planes, sonars, radars, and more advanced tanks were being used. Fighter planes allowed soldiers to kill more people faster, with more destruction then ever before. Bomber planes allowed them to destroy enemy base in no time but at the same time causing destruction to the civilians. Consequently causing a total war where everyone gets involved. With the invention of Sonars and Radars, the soldiers were able to detect enemy planes, tanks from miles away, but also let to sneak attacks. (60 million casualties/3% of world population at the time)