Time and again, when a power tries to dominate Europe, it is forced to create a coalition of armies. There are currently 50 nations in Europe and many more ethnic groups. For any one power to dominate them all is a major task.
Two thousand years ago, a small city-state in Italy was able to dominate the Mediterranean only because it provided a way for non-Romans to become citizens and allowed them to join the army. Even then, it had to ally with Arabic tribes to the east and Germanic tribes in the north and west to secure its frontiers.
Much later, the Habsburg Empire was famous for its ability to create marriage alliances.
Napoleon Bonaparte took military cooperation to a level not seen since the days of Rome in his attempted takeover of Europe, most notable in his invasion of Russia. In that campaign, half of the infantry in his invasion was non-French.
“It was certainly the largest invasion force in the history of mankind to that time, and very much a multinational one,” Andrew Roberts writes in Napoleon the Great before listing 20 states that contributed to the army. “Much has been made of the breadth of the seven coalitions that Britain brought together against France during the Napoleonic Wars, which is indeed impressive and significant, but the broadest coalition of all was this one that fought for France against Russia.”
What the European Union is trying to do today is certainly not unprecedented. To successfully project power into Russia or the Middle East, European powers have always required some kind of coalition. Now that Europe is looking to get more involved in the Middle East and North Africa in order to confront radical Islam and the migrant crisis, it will need to cooperate.