A cheap currency helps finance the US tech boom. Washington is intervening because allies matter only when they are useful

Japan’s yen has slid back towards 160 to the dollar, despite last month’s extraordinary US-Japanese intervention. Traders are betting that its role as a cheap funding pipeline for global finance will continue. Earlier this month the Trump administration stepped in to help Tokyo stabilise its currency. “Japan’s been very good to us, with the exception, of course, of Pearl Harbor,” Donald Trump explained. More help is likely to be on its way.

But this is less a rescue of the yen than an attempt by Scott Bessent, the US treasury secretary, to preserve a cash spigot that benefits the US. Japan’s ultra-cheap money has become a global funding utility: bankers borrow yen, sell them for dollars and buy higher-returning US assets, notably tech shares. Rising American stock markets support collateral and investment. Japan’s “carry trade” is one of the reasons Wall Street can lever hundreds of billions into AI. Research suggests AI sucks up more than 1% of US GDP.

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