How and why did america become the policeman of the world?
Answers
America is not the world's policeman.
The behaviour of the USA in projecting its power around the world is specifically to promote US interests. These interests coincide with those of some Multinational Corporations. The “policing ” that you feel is being undertaken is selective.
The role of major power projector was taken over from France and Britain after both those nations lost control of their interests overseas. The money these two countries wielded was as a result of the large trade organisations embedded in their societies.
After WW2 both nations were beggared and the control of the wealth drifted to the USA. Because these nations had other priorities and lower cash flow their armies, navies and empires began to shrink. At the same time Multinational Corporations came into existence because they no longer existed within one nations zone of control.