After WW2, a number of international institutions were formed to steamline the flow of trade across borders. It was feared that without agreed and structured intergovernmental bodies independent of political tinkering, European countries would raise their protectionist barriers (tariffs) and seek to keep foreign competition from hurting their own industry (and so, their own people). This marked the formal creation of an international economic system.
Free and unhindered trade was (and remains to be) seen as a vital component for recovering degraded economies. PIIGSty gives you a summary of the 3 main institutions, all of which remain in operation today.
Heres the PDF PIIGSty Econ 101 #32 International Eco Sys