For the first time since 1998, the United States and Japan intervened together in currency markets to arrest the yen's slide to forty-year lows — a partnership that speaks less to altruism than to the quiet interdependence of the world's two largest holders of sovereign debt. Washington's willingness to act was shaped by a sobering recognition: a Japan forced to sell its vast Treasury holdings to defend its own currency could unravel American bond markets already straining under rising yields. In this moment, the fates of two economies revealed themselves to be more entangled than either natio