^Well, the EU already has a greater economy than the US, and over the past few years (excluding this recession), EU growth has been greater than that of the US, and when the other nations join, (such as Turkey) that gap is going to grow.
However, it won't get much clout until it becomes more unified on economic and foreign policy. Economic policy is a gradual thing, and the Lisbon treaty worked towards greater foreign unity, but that's been stalled.
Intiatives such as SAFE are the building blocks towards an EU Army, but that'll never be the size of the US's, considering that, even now, the EU's combined armies only equate to under half of the US's, and a unified force will probably be smaller.







