After two total and all out wars the countries had realized that the absolute chaos of a war would bring the world to collapse if aggressive policies were still relied on.
Step-by-step explanation:
The two world wars that happened in a span of 20 years had ruined the economy of the European nations and brought the countries to a virtual stand still with the loss of millions of young men.
The war brought destruction to even the winners enough to not justify all out conflicts anymore.
The presence of nuclear weapons by the end also brought the reality of a nuclear war to the fore as there would always be a possibility of nuclear attacks if wars escalated.
The conflict would prove to be too costly if it ever happens.