The creation of an EU army could harm diplomatic relations with the USA  We are completely ignoring the issue of geopolitics and how the creation of this entity would be a direct move to replace NATO as the primary defender of Europe. This would of course mean a rejection of the US, as the heart of NATO. What would follow from this would be an extremely unpredictable and volatile place to practice international relations. One thing that we can predict, however, will be the ‘cold shoulder’ the US would suddenly show the EU. The US would feel as if its ally had used it to gain strength after WWII (The Marshall Plan), and now that it’s back on its feet again can forget and even challenge America’s supremacy.