For years, successive governments treated defence as the budget that could always wait. Equipment programmes were delayed. Stockpiles were allowed to shrink. Recruitment faltered. The assumption was simple: major war in Europe was a relic of the past, and Britain could safely cash in the so-called peace dividend.
That assumption has collapsed.
The problem is not simply that Britain has spent too little. It is that the nature of defence has changed far more quickly than the way we fund it.
Political debate remains fixated on percentages. Should Britain spend 2.5%? Three per cent? Three and a half? Those figures dominate headlines, but they tell us almost nothing about whether the country is actually becoming more secure. A percentage does not produce capability.
It does not recruit soldiers or retain experienced personnel. It does not replenish ammunition stockpiles depleted by years of underinvestment and support for Ukraine. It does not build frigates, modernise air defences, strengthen cyber resilience or protect the cables lying on the seabed that carry almost all of Britain's digital communications. Those capabilities take years to build and decades to sustain.
The world our defence budget was designed for no longer exists.
At the same time, entirely new fronts have emerged.
The Armed Forces must deter Russia, meet NATO commitments, maintain the nuclear deterrent, patrol vital sea lanes and respond to crises overseas. At the same time, the country must invest in cyber security, artificial intelligence, electronic warfare, missile defence, resilient supply chains, intelligence capabilities and the protection of critical infrastructure.
Every one of those demands resources.
The challenge, then, is not simply spending more. It is recognising that defence has become the operating system of the modern state. Without secure trade routes, there is no economic growth. Without resilient digital infrastructure, public services cannot function. Without secure energy supplies, industry slows. Without credible deterrence, diplomacy carries less weight. Defence no longer sits alongside prosperity. It makes prosperity possible.
Military capability cannot be created overnight. Shipyards cannot suddenly produce more warships because a crisis has begun. Ammunition factories cannot be switched on at the first sign of conflict. Engineers, pilots and submariners require years of training. Once those skills are lost, rebuilding them becomes vastly more expensive than preserving them in the first place. That is where Britain now finds itself.
It demands a different understanding of national security.
The question is no longer whether Britain can afford to spend more on defence.
The question is whether Britain can afford another decade of funding national security as though the strategic environment has remained unchanged.