Donald Trump and nuclear weapons are a scary mix. As president, he greatly expanded the US nuclear arsenal, scrapped arms control treaties and repeatedly threatened to start a nuclear war. On leaving office, he stole nuclear secrets from the White House and leaked their contents. A judge recently questioned his mental health.

For close ally Britain, the scariest thought is that Trump, if re-elected in November, could fatally undermine the UK’s “independent” nuclear deterrent, or worse, pressure London into actually using it. If Trump blundered into a nuclear showdown with, say, China, Russia or North Korea, Britain would be expected to back him – and could become a target.

None of these scenarios may be ruled out, despite UK insistence that it retains sole operational control of its four Vanguard-class nuclear missile submarines. In truth, such outcomes grow more plausible as the international security situation deteriorates, Trump threatens to abandon Nato and Europe, and nuclear arms proliferate globally. Successive UK governments are primarily to blame for Britain’s deepening nuclear nightmare. All have colluded in the pretence that the UK deterrent, known generically as Trident, is independent. In fact, the Vanguard submarines rely on American technology, logistics and maintenance, as will their Dreadnought-class successors. The new W93 replacement warhead borrows from US designs.

Even the US-made Trident II D5 ballistic missiles that carry the warheads are........

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