‘There is only one thing worse than fighting with allies, and that is fighting without them’, said Winston Churchill near the end of the Second World War. An alliance in the strict meaning of the term is a commitment between states to support each other in war. Europeans and Americans will immediately think of NATO and collective defence: a long-term if not permanent alliance to defend each other in case of aggression against a fellow member. But an alliance can also be concluded for a specific short-term purpose; and that purpose can be to make war. The great powers need allies and partners in all dimensions of international politics though, not only in the military sphere. They seek partners who will work with them to bring their particular view of the world order into practice, for example by voting along with them in existing international organisations, by joining them in creating new multilateral institutions, and by aligning their economic practices.

If one is in an alliance, one must consult one's allies, decide together, and take action together when the purpose of the alliance is at stake. That inevitably takes more time than when one acts alone, and may lead to compromise decisions. After the 1999 NATO air campaign against Serbia, to force an end to the violence in Kosovo, the US complained of the inefficiency of such a ‘war by committee’. But allies bring additional resources, and may also add legitimacy to one's actions: the more support a strategy garners from other states, the more acceptable it will appear to the domestic and international audience. When US President Barack Obama created the global coalition against ISIS in 2014, from the start he sought the participation of a number of Arab countries in order to create the perception of a truly international rather than a western initiative, and he consciously avoided the label of the US’ standing alliance, NATO. Eventually, the coalition recruited 82 members – not out of military necessity, but for political reasons.