The Pentagon’s latest fiscal fantasy, a $1.2 trillion missile defence shield dubbed the ‘Golden Dome’, has sent a shiver down the spines of British defence planners. On paper, it promises to protect the US homeland from intercontinental ballistic missiles. In practice, it signals another worrying pivot towards American isolationism, leaving the UK and other NATO allies dangerously exposed.

Let us be clear: this is not a charitable contribution to Western security. It is a monument to American paranoia, a vast expenditure that will suck capital from more productive uses across the Atlantic. The Treasury will cheer the domestic spending boost, but the City should brace for a new source of volatility. Gilt yields may well catch a bid as UK defence spending is forced higher to fill the gap.

For decades, the UK has relied on the American nuclear umbrella and integrated missile defence. But the Golden Dome, by design, prioritises US soil over allied skies. It is a physical manifestation of the Trumpian worldview: America first, everyone else second. The message is clear: you are on your own, chaps.

This comes at a particularly inopportune time. British defence budgets are already strained, with commitments abroad and an ageing Trident system that will require billions in upgrades. The fiscal arithmetic is grim. To maintain credible deterrence, the Chancellor will have to raid other budgets or issue more debt. The bond vigilantes are watching.

Moreover, the Golden Dome could spark a new arms race. Russia and China will view it as a threat to their second-strike capabilities, prompting countermeasures that make the world less safe. A more efficient approach would be to invest in shared early-warning systems and cooperative missile defence, but that requires trust and multilateralism, which are in short supply.

The market reaction has been muted so far, but defence stocks on both sides of the pond are rallying. Lockheed Martin and Raytheon are obvious winners. BAE Systems, with its strong US exposure, may also benefit. But the UK’s own defence industrial base risks being sidelined as Washington prioritises domestic suppliers.

In the end, the Golden Dome is a classic example of government overreach: a monument to fear rather than a rational allocation of resources. It will do little to make the West safer while saddling American taxpayers with a colossal bill and leaving allies in the lurch. The bottom line is that the UK must now spend more on defence, or accept a diminished role on the world stage. Neither option is palatable, but both are now inevitable.