When the centre-left French politician Aurore Lalucq posted a warning last Wednesday that Donald Trump could cut off Europe from international payment systems, the clip went viral. To many, her message made sense. After all, if Mr Trump was prepared to test allies’ boundaries over Greenland, it is not far-fetched to imagine Visa and Mastercard becoming used against a recalcitrant Europe. The US can turn off payment systems it controls. Russia learned this first-hand after sanctions were rightly applied for its invasion of Ukraine. As up to 60% of Russian…