Collective defence is the principle that an attack on one NATO member is considered an attack on all. Article 5 of the Washington Treaty formalises this commitment. Its importance is mainly psychological and strategic: it raises the cost of aggression against any member by making clear that an attacker would face the combined response of the alliance. It has only been invoked once, after the September 2001 attacks on the United States, which demonstrated both its significance and its political complexity.

It has been a long-standing source of tension. The agreed target is for member states to spend two percent of GDP on defence, but many have fallen below that for years. The United States has consistently argued that it bears a disproportionate share of the alliance’s costs. That said, defence contributions are not purely financial. Some members provide critical geography, intelligence, or niche capabilities that are harder to measure. In my view, the debate over percentages can obscure what actually matters, which is whether the alliance can deploy effective forces when needed.