US diplomatic and military links encourage America’s allies to hold dollars. States with their own nuclear weapons hold fewer dollars than countries that depend on the US for their security needs. Being in a military alliance with a reserve-currency-issuing country boosts the share of the partner’s foreign-exchange reserves held in that currency by roughly 30 percentage points. The evidence thus suggests that the share of reserves held in dollars would fall appreciably in the absence of this effect. . . .

South Korea and Japan are thought to hold about 80% of their international reserves in dollars. One can imagine that the financial behavior of these and other countries would change dramatically, with adverse implications for the dollar’s exchange rate and US borrowing costs, were America’s close military alliances with its allies to fray.

Nor is it hard to imagine how this fraying could come about.