An oft-deployed argument in favour of further centralization and integration of Europe is based on international relations. Only Europe as a whole can compete with Great Powers like the USA, Russia or China. If not speaking and fighting with one voice, it even threatens to pale in comparison to emerging countries like Brazil or India. For this reason, Europe needs an army and to follow a Common Foreign and Security Policy.
However, is this true? For sure, France, Great Britain or Germany alone cannot compete any more with their military, let alone other European countries. Nevertheless, there always has been the possibility of alliances. With the North Atlantic Treaty Organization, most European countries are militarily united in a highly successful way. People arguing against the dependency on the US should not forget history. Although European countries certainly have their own, sometimes different interests, the US has proven, and will further prove, a valuable partner, regardless of minor differences.
Countries however, are free to choose with whom they ally themselves. In this period of peace in the western world, military issues should not play a big role. Economic interdependece, through things like free trade agreements, will secure peace much more efficiently than a spiral of armament. Europe's troops are its merchants, not its soldiers.
With regards to a Common Foreign and Security Policy, this only makes sense if all European countries have common interests. This is not the case for historic and cultural reasons. Countries with former colonies will always have different interests than countries with none. France, for example, still conceives itself as the protecting power for Mali, because French companies exploit resources there. In contrast, Germany or a lot of small European countries have no connection at all to this country, but even now get roped into it more and more. Although one might not forget the terrible humanitarian situation in this country plagued by civil war, France's onesided interventions create - like every intervention does - more problems than it solves.
Regarding Europe, as a Great Power in military issues, it would have to intervene in other countries to embrace its role. Does the world really need another policeman fighting for democracy, civil rights and freedom? Especially considering Europe's own deficits in this regard. The answer is negative. The military budget of a common army already devours enormous amounts of money. Humanitarian interventions, regardless of their purpose, will cost much much more. Not to forget the higher cost for any 'European' army members involved in these interventions.
Furthermore, a Common Foreign and Security Policy would shift further competences into the European decision-making arena. National parliaments probably would be disentitled from deciding their own foreign policy. Although democratic decisions are far from perfect in national parliaments, they are probably still better settled there than by European technocrats. Even if the European Parliament does decide these issues, there will still be problems. Due to diverging interests, a consensus will be highly implausible. Is a majority decision then justified to legislate about the fate of a people? What about the minority countries – will they support the decision in action?
This, last but not least, is a practical reason. Can you build a highly efficient army lacking a common language, culture and mentality? Even though diversity is usually a benefit, this mix might be controversial.
In conclusion, a Common Foreign and Security Policy for Europe has its problems. The fantasies of being a new Great Power, on a level with the US and China, could cause much evil. Europe's destiny should rather lie with peace than war. Economic interdependence and decentralized alliances will serve peace far better than playing world policeman. After twenty centuries of war caused by European nations, they should know better by now.