THIS week, as The Economist went to press, an American F-35 Lightning jet was due to sweep out of the sky over Chesapeake Bay in Maryland and onto the deck of a waiting aircraft-carrier. This is nothing out of the ordinary for Pax River, the nearby air base that has housed the American navy’s test-pilot school for over 70 years. But the ship in question is the HMS Queen Elizabeth, Britain’s largest-ever warship, and this will be the first landing on a British carrier in eight years.

Neither ship nor plane is entirely new. The carrier began sea trials last summer and Britain took delivery of its first F-35 six years ago. But their integration marks an important moment not only for the Royal Navy, which resented gibes that it had built an aircraft-carrier with no aircraft, but also for British air power, which is celebrating the centenary year of the Royal Air Force.

Though the F-35 has been plagued by cost overruns and technical snags, it is a path-breaking aircraft that comes into its own when dodging enemy air defences. The significance of this feature was underlined on September 24th, when Russia vowed to send powerful new radar and missiles to its Syrian ally. Kaliningrad, the Russian enclave between Poland and Lithuania, bristles with even more advanced anti-air systems. If the 800 British troops deployed to Estonia were to run into trouble, pilots sent to their aid would be grateful for the F-35’s stealthy profile. No less important is that the plane can act as a flying intelligence hub, pumping information back and forth between ships and aircraft.

The debut landing is also a reminder of the deep co-operation between British and American armed forces, despite the chaotic state of the wider transatlantic relationship. The American jet landing on the Queen Elizabeth was due to be piloted by a Briton, and plucked from a common pool of F-35s being put through their paces by a joint test team. British pilots have flown off American decks throughout the Royal Navy’s eight-year carrier hiatus, keeping their skills sharp. Mark Sedwill, Britain’s national security adviser, acknowledges that the Queen Elizabeth would “inevitably” be deployed alongside allied ships in any serious conflict. This military intimacy explains why British ministers have been more circumspect than their European counterparts in proclaiming the death of the Atlantic alliance.

But the F-35’s arrival also raises new questions about Britain’s defence budget. Britain originally envisaged a fleet of 150 jets. That number has already fallen to 138. It could be squeezed further. In the summer Gavin Williamson, the defence secretary, set out plans for a new combat aircraft, the Tempest, to replace the 15-year-old Eurofighter Typhoon. These jets have different strengths. The F-35’s forte is sneaking past air defences and attacking the ground; the Typhoon, and perhaps the Tempest, are optimised for combat in the air. But the Treasury is interested in trade-offs, not the finer points of aerial warfare.

By around 2035, Britain will be paying for the last F-35 deliveries, the first of the Tempests, and eking life out of an ageing Typhoon fleet. “Something will eventually have to give,” says Justin Bronk of the Royal United Services Institute, a London think-tank. He warns that the Tempest is unlikely to be viable unless defence spending is increased significantly, or F-35 orders pruned.

This would not only infuriate President Donald Trump, who views arms purchases as something akin to protection money. It would also stretch Britain’s air-power thin. Last year Boris Johnson, then foreign secretary, promised to send “the two new colossal aircraft-carriers that we have just built” straight to Asia. But there will be demands on their time in the Mediterranean, the Gulf and the Indian Ocean. The government must choose between stocking up on F-35s for the air force and navy, lean and hungry after years of cuts, or investing in a new generation of air power, shrouded in the fog of future war.