The real test of the Defence Investment Plan is endurance

With the plan now expected before July’s NATO summit, the debate has settled on which programmes will survive and which will slip. The more demanding question, and the one most of us keep returning to, is whether the forces those programmes produce could actually be kept fighting in a serious war.

The government intends to publish the Defence Investment Plan before NATO leaders meet in July. Promised since the Strategic Defence Review landed last June, it has been put back more than once, and the defence readiness minister, Luke Pollard, told Parliament this month that the Prime Minister was “determined” to publish before the Ankara summit on 7 July.

When it arrives, most of the attention will follow the money, and which of the headline programmes survive intact. The questions across Whitehall concern whether GCAP, the next-generation combat aircraft, is delivered on schedule or pushed to the right, what becomes of the surface fleet and the Army’s numbers, and how the AUKUS submarine commitment is to be paid for. The former defence secretary Ben Wallace has predicted the Treasury will fudge it, arguing it cannot fund submarines, a new combat aircraft and the Army at once, and that the likely result is a slow hollowing out dressed up as a plan.

There is little reason to doubt his reading of the politics. The most probable path, in my admittedly outsider view, is partial funding: the nuclear deterrent and GCAP protected, money found for munitions and drones, the savings taken from conventional equipment and personnel through stretched schedules and a trimmed order book. The result would be a two-tier force, a few exquisite platforms above noticeably thinner provision.

This focus on which programmes survive obscures the more important lesson of the war in Ukraine. Acquiring equipment is the comparatively straightforward part of building military power; sustaining it through a prolonged, high-intensity fight is far harder, and far more expensive.

After two decades of expeditionary campaigns fought by small, high-end forces, the prevailing view now is that quantity carries a military value of its own, since a war in which shells are expended in the hundreds of thousands and drones consumed almost as readily as ammunition turns as much on endurance and replacement as on manoeuvre. Ministers have absorbed this, and now speak fluently of scale, of an Army made “ten times more lethal” and of fielding drones at the front in significant numbers.

That instinct is correct, since a force unable to generate weight of fire or replace what it loses offers no real deterrent to a capable adversary. The difficulty is that the mass on paper is not the mass that can be generated and maintained in the field, where Britain has been losing ground for some years.

Malcolm Chalmers of RUSI has given the condition a name now in common usage, the “hollow force”, meaning a military that presents well on paper while lacking the endurance, stockpiles, industrial resilience and mobilisation depth needed to keep fighting once a conflict is under way. The Royal Aeronautical Society put it more sharply in its implementation-gap assessment, judging the realistic danger to be less a sudden collapse of capability than a slow erosion of depth and resilience, which is the hollowing out the review set out to reverse. It noted that, were Britain required to generate high-intensity air operations within six months, it would be critically short of trained F-35 pilots and the RAF would struggle to sustain them.

Munitions are where this hollowness is felt first, and RUSI’s work on magazine depth is candid about how long any remedy will take. Because successive governments let the stockpile run down, it will take successive governments to rebuild it, since production cannot be surged at the moment of crisis. Complex weapons in particular rely on long, fragile supply chains, including the MBDA-led missile teams, that cannot be conjured up at short notice, and the most sophisticated could not be replaced within a year whatever the budget.

The government has, to its credit, recognised the problem. Last year’s Defence Industrial Strategy committed around £1.5bn to munitions and set out an ambition for “always on” production, with half a dozen new energetics factories meant to keep the lines warm between orders. The instinct is sound, but “always on” works only for as long as the orders keep flowing; a single injection followed by a quiet drawdown would leave industry where it began, unwilling to invest in capacity whose future use it cannot count on.

Personnel are a related difficulty, at once the least glamorous line in any budget and the first to be raided when savings are demanded. RUSI’s analysis of how to deliver mass for the British Army argues that genuine depth means more than the front rank, requiring a second echelon to follow and the structures to generate a third, and concludes that the review and the Treasury cannot fund extra regulars alongside the equipment programme and the munitions at once, which leaves a properly resourced reserve as the only affordable way to add it. Sustainment also reaches well beyond fighting troops to the spares, maintenance, fuel and trained crews that decide whether a fleet is available or stuck in a hangar awaiting parts.

All of this returns to the money, and to the reported £28bn shortfall driving the standoff between the Ministry of Defence and the Treasury. When the books cannot be balanced, the temptation is to protect the flagship programmes that carry headlines and industrial jobs while pushing the less visible sustainment into later years, to become a future government’s problem. One independent modelling exercise framed the trade-off well, observing that spending too little does more lasting harm than spending too much ever undoes, since a hollow force forms far faster than a capable one, so plans that defer the hard spending look convenient now and prove costly later. The point is sharper still given NATO’s benchmark of 3.5% of GDP for core defence, and 5% once wider security spending is counted, which presses the UK to do more just as its own affordability gap is widest.

The plan should be judged less by the platforms it lists, or how its headline figure compares with allied nations, than by what it provides for the capabilities that decide a long campaign: deep magazines, replenished stockpiles, warm production lines, spares, and enough trained people to sustain a fight well beyond its opening weeks.

Mass will always attract the headlines and the political credit, yet it is the ability to sustain a force through a prolonged conflict that decides whether it can fight at all, and a plan that defers that sustainment to protect its flagships will leave Britain with armed forces that look formidable on a published chart yet would be exhausted within weeks of real combat. The more honest measure of the government’s ambition is the endurance it is prepared to pay for, and that, rather than the length of the procurement list, is the figure a capable adversary will trouble to study.

This article is the opinion of the author and not necessarily that of the UK Defence Journal. If you would like to submit your own article on this topic or any other, please see our submission guidelines.