What were the fundamental causes of the spectacular prosperity of the untied states in the decades immediately after world war ii?

The war created jobs and the return of the soldiers created a population increase.

The need for goods during war fueled the economy by creating jobs and growing the manufacturing business. When the soldiers returned they reunited with their families and caused a "baby boom" the increase in population created a higher demand for goods.

Democracy and darwinism would be my answer.

Government taxes ? i guess