During the Cold War, deterrence was treated as a settled doctrine: make aggression costlier than its potential gains. For three decades, Western militaries reduced forces and stockpiles, prioritising expeditionary operations over territorial defence. Russia’s invasion of Ukraine exposed the limits of that approach. NATO is now rebuilding its doctrine to meet the threat.
From tripwire to forward defence
The old posture rested on a brutal simplification: a thin covering force would trip the article 5 guarantee, and the full weight of the alliance would follow. It was cheap, and it was credible only as long as everyone believed the guarantee would actually fire. After three years of war in Ukraine, allies have concluded — at different speeds — that they must hold the line at the border, not after it.
The consequence is the largest peacetime restructuring of the alliance’s force posture since the 1990s. Pre-positioned equipment, reinforced brigade structures in the Baltic states and Poland, and a fundamentally larger German army than anyone planned for in 2015. The numbers are striking in aggregate: European allies now spend a combined share of GDP on defence that would have been politically unthinkable a decade ago.
The credibility problem has moved upstairs
Yet force structure was the easy part. The harder problem is the one the Cold War alliance never had to solve in public: what actually happens in the first hours after an attack, when a decision to trigger collective defence involves thirty-two governments, at least two of them nuclear-armed, and a decision cycle measured in minutes if the attack escalates.
The alliance’s answer has been institutional: faster decision procedures, lower activation thresholds, and a new generation of national contingency laws that shorten the path from political decision to operational movement. But deterrence ultimately rests on what an adversary believes, not on what an alliance posts in its rules of procedure. And here the record is mixed. The invasion of Ukraine showed deterrence failing against a non-member; the question that now haunts the eastern flank is whether it would hold for a member.
Spending is now structural
The headline story of the past years has been the return of the 2 percent target from aspiration to floor. The quieter story is why the increases are likely to persist: the European defence industry cannot meet current demand, munitions stockpiles are far below doctrine requirements, and the demographic clock means armies must spend more per soldier simply to maintain readiness. These are structural costs. They will not disappear when the war in Ukraine ends — the war has accelerated a rearmament that the continent would have had to make anyway.
The alliance is relearning a doctrine it once believed it had mastered, and doing so under fire. The mistake would be to assume that because the spending is up and the forces are forward, deterrence has been restored. It has not yet. It is being rebuilt, and the next few years will determine whether the rebuilding outpaces the adversary’s own adaptation.
