Although this is a very broad question, we can generally say that the process of creating free trade between nations (by reducing trade barriers) has been going on for centuries. It is also proper to say that the process has accelerated greatly since World War II.
Up until around 250 years ago, there was little in the way of free trade because nations held to the idea of mercantilism. This belief slowly eroded over the years. After WWII, there was a much greater push to lower trade barriers both to create more economic activity and to promote closer ties between nations. This can be seen in the creation of such agreements as the GATT and such bodies as the WTO.
Today, the impetus seems to be towards more free trade and fewer trade barriers, but the process is by no means complete.