National governments of Europe have always had allies they could look to for support when faced with threats to their national security. Today every European state belongs to dozens of multi-national organisations that differ in their purposes and in what they can add to the effectiveness of a national government acting on its own. Informal alliances such as the special relationship of Britain with the United States lack an institutional framework, but can still facilitate co-operation on security services. Democratic European governments need popular support to meet the costs that belonging to an alliance imposes. Whereas the government of the day’s choice of allies commits all of its citizens, public opinion differs about whether allies are desirable in principle and if so, which ally would be best for dealing with a particular security threat: the EU, NATO or the UN. While there is majority support for defending security through alliances, Europeans do not rely on a single ally but pragmatically switch preferences between allies as the security threat changes.