Burden Sharing
The distribution of an alliance's costs — chiefly defense spending — across its members, and the free-rider problem that arises because collective defense is a public good: once an alliance deters an attack, every member benefits regardless of its own contribution. NATO's 2 percent GDP guideline (informal since 2006, given firmer political weight at the 2014 Wales Summit) has no binding enforcement mechanism — only the reputational cost of being named in NATO's own reporting — which is exactly why compliance was gradual and partial until Russia's 2022 invasion sharply shifted threat perception and, with it, actual spending.