The common economic interest was always seen as the best key to persuade countries to eventually federate. Read the arguments from early leaders of the ECSC and

The common economic interest was always seen as the best key to persuade countries to eventually federate. Read the arguments from early leaders of the ECSC and it's all very plain. The vision was always to reach a full political union, through the lever of an economic union that is necessary to remain competitive with the US and (the USSR back then, but today) China. Because in a superpower world, European countries alone are utterly irrelevant. You can take any trade bloc out there - none has the influence or the prospects of the EU, they are all fundamentally weak. You need shared political force to make a difference.