Answer:
Guns became less efficient.
Step-by-step explanation:
One Bullet shot by guns might not kill a fully vested soldier. One drop of atomic bomb will kill hundreds of thousands of people.
The atomic bomb that dropped By United States in Hiroshima and Nagasaki killed more than 200,000 people . This the atomic Bomb that's developed in 1945 and effectively ended the world war II.
In 2020, the development of nuclear technology far surpassed what we had in 1945s. Which made the use of guns almost obsolete.
If another world war happen in the future, the use of Planes (as medium of transportation) and Chemicals warfare's will be the most efficient methods to destroy the opposition.