One tank that is not on its way to Ukraine is Britain’s latest super-tank, the Ajax. It has been more than 10 years in the planning and £5.5bn in the building, but it doesn’t work. Its tracks suffer from speed bumps and its interior keeps injuring its drivers. If I were the god Ajax, I would sue for my reputation. It is nearly a quarter-century since Tony Blair unveiled his doctrine of so-called humanitarian interventionism. This was in support of the UN’s grandiose declaration of a “responsibility to protect” human beings everywhere. As Margaret Thatcher had liberated the Falklanders, so Blair would liberate Kosovans, Iraqis, Afghans and Sierra Leonians. The UK now has military bases in 42 overseas countries, from Oman to Mali, from Kenya to Belize, an outreach shared with no other European state. An ever-powerful defence industry lobby has since kept British defence spending at around 2% of GDP, supposedly an indication of military might sufficient to deter an aggressor. This deterrent is now being tested by Vladimir Putin. His attack on Ukraine is presented by hawks as a threat to Nato and the entire west. Though the nuclear deterrent is holding, that of Nato’s implied conventional response did not, at least a year ago. Putin appears to have validated decades of warnings by defence historians that the cold war will never be over. Russia must be seen as an ever-present enemy; Putin agrees. Just as Nato risked confrontation in advancing east in the 1990s, he felt justified in advancing into Chechnya, Georgia and Donbas. It is not so much cold war as old war and it has left Whitehall floundering. Is defence really about global influence or rather about Nato solidity – and if the latter, how should the money be spent? Britain’s procurement of extravagant weapons feeds the vanity of politicians of all parties . Aircraft carriers were supposed to be phased out in the 1960s with the end of empire. Blair authorised their reinstatement in 1998 for expeditionary power projection (after a defence review on which I served). David Cameron refused to cancel them in 2010. Johnson recently sent one of these dinosaurs, HMS Queen Elizabeth, to the South China Sea. Another, HMS Prince of Wales, was meant to help boost trade down the east coast of America. It broke down at sea and returned to port. Both have to use US planes, with US permission. In the same company is the Ajax armoured vehicle, the Lightning fighter jets, the £31bn Dreadnought submarines, Project Mensa for nuclear infrastructure and the Morpheus military communications system. One follows another into wild overspend and delay. For a medium-sized European power this is beyond parody. Each year the defence secretary has to plead or bully Downing Street for more cash while their department shows incompetence that would have an NHS trust in special measures overnight. It is hard to quarrel with the defence analyst Paul Rogers’ description of these weapons as no more than “delusions of post-imperial grandeur”. Every country needs defending, even one as ostensibly secure from territorial threat as the UK. This operates at two levels. It means an army aiding civil authority in a crisis, be it riot, terrorism or pandemic. Covid saw 20,000 soldiers on stand-by. Beyond that, an army is needed to support alliance activities, primarily Nato. Here the commitment is more discretionary. That Russia’s invasion of Ukraine constitutes a true existential threat to all of Europe is absurd. But it certainly threatens frontier states to which Nato boldly offered absolute security guarantees. Either way it must cause those, like myself, who saw no point in nuclear deterrence, to pause. To what extent Britain should be tooled up for a land war in Europe is a dilemma that any non-expert must find hard to resolve. One thing is for sure: for all the glamour of ships and planes, it is armies that matter. But what matters even more than their size is the politics of their deployment. Two years ago, the US soldier and academic Sean McFate dared to ask Why the West Doesn’t Win Wars. His answer lay not in soldiers or their equipment. It lay in the decisions – so often bombastic, reckless and thoughtless – that drove them to war in the first place. Putin’s war on Ukraine has shown the same weakness as did the west’s war against the Taliban: that no force is more powerful than the will to defend one’s own soil. Largely for this reason, it has been estimated that four wars in 10 are won by militarily the weaker side. The British government has decided to cut its army by nearly 10,000 troops. No layperson can judge what impact this may have on operational effectiveness – let alone deterrent value – because the “operation” is conjectural. It still leaves a force of 73,000 ground troops. There is talk of this being insufficient to take over from Germany in leading Nato’s high-readiness command. It has left the Tory defence select committee chairman, Tobias Ellwood, calling it “a reduction in our hard power that would diminish our international standing and ability to sway opinion on the world stage”. I could say the same of Brexit. What shines through these arguments is a divergence between these images of 20th-century conflict and what defence may mean in the future. Russia has been proved weak on the battlefield, but not in its cyberwar, domestic brainwashing, diplomatic manipulation and resistance to sanctions. Its hold is unyielding on Iran, Syria and certain countries in Africa. Threats are also changing. The seas have become as permeable as the skies. Unmanned weapons enable armies to go further and be more reckless. Drones are transformational, except in occupying territory. Even here soldiers are being supplanted by mercenaries. The Russians use prisoners most effectively. Half the Americans serving in Iraq and Afghanistan were contractors. Much in today’s defence debate hinges on how a Labour government will respond. It has long been terrified of cutting defence, of being portrayed as weak or appeasing. At very least Keir Starmer may snap out of Britain’s “post-imperial” delusion. Modern defence should be what it says on the tin: about defence. The defence secretary, Ben Wallace, has recently spoken of taking it “budget by budget”, seemingly de-escalating the arms race. He sounded at least common sensical. Perhaps for the first time in decades, Britain’s defence has what it most needs: a secretary of state for sanity. Simon Jenkins is a Guardian columnist