The Holy Prophet (s) said that at the time of the Messiah the burden of war will be lifted. World War One led to such carnage that it was thought that there would never be another war and the League of Nations was set up.
But World War II was even more devastating than World War I. This time whole cities were carpet bombed. During World War I devastating weapons were invented. During World War II even more terrible weapons were invented such as the atom bomb and later the Hydrogen Bomb.
Use of the Atom Bomb on Japan showed that if there is another World war there is a real possibility that the whole world will be destroyed.
This is another sign that Hazrat sahib was the Promised Messiah because it was after he appeared that all people realised that they can no longer afford to go to war.