The US is perhaps strong enough to withstand the rigours of monetary tightening. It is less clear whether others are so resilient. The risk is that rising borrowing costs in the US will set off a worldwide margin call on dollar debtors - or a "super taper tantrum" as the IMF calls it - that short-circuits the fragile global recovery and ultimately ricochets back into the US itself. In the end it could tip us all back into deflation.