The only interest that is at stake here is that of a perceived common external military threat. If that is agreed upon then that is enough, simply sign a Treaty to that effect or support one previously committed to. As to internal security, law and compromises work only by mutual consent. When that fails you are faced with the worst of all wars, a civil war. I contend that is far more likely to happen if nation states surrender their sovereignty to a union of member states. That is why such unions need a huge army, as you put it. As to law and compromises working between countries, best to carry a big stick and speak softly. Unless of course you are a believer in World Government, in which case please refer to comments above re Unions.