The development of the European Union was due to the travesties left behind via World war one initially, which resulted in the treaty of Versailles (1919) and was a peace settlement, signed only a year after the first world war, in an attempt to not only hold Germany and her allies to ransom but responsible, weak, in check and to punish them financially.
The anger and resentment that built up in Nazi Germany as a result of the heavily imposed precautions set in the treaty of Versailles 1919 was part of the long term cause, which directly contributed to the cause of the second world war, short term causes such as Austria and Czechoslovakia in the 1930 also had a hand. Europe was crushed, due to these events as those involved underwent fatalities way beyond anything ever imagined
So the need to prevent any further atrocities was seen as important so binding Germany's coal and steel industry (the strength of its war machines), into an economic and political alliance. Was of major importance
The Soviet Union also a concern due to occupancy of the former East Germany, Eastern and Central Europe, urged Western European and America to come together and form NATO (North Atlantic Treaty Organisation) 1949, the former West Germany, joined Nato in October 1954 and along with others received millions of dollars, under the marshal plan to support economic recovery.

Restricting Germany’s and France coal and steel was a sure fire way to encourage a plan, much than before, to give all parties concerned confidence that any war between the Europeans would now be avoided, hence the words famously repeated by many:
“It is to re-create the European Family, or as much of it as we can, and provide it with a structure under which it can dwell in peace, in safety and in freedom.
We must build a kind of United States of Europe.
In this way only will hundreds of millions of