As Javier Milei’s reform agenda is forging ahead in Argentina’s legislature, a currency war is emerging in a left-leaning northern province to defy his austerity program. It underscores the need for Mr. Milei to move quickly on his monetary vow, starting with stabilizing Argentina’s debased peso. The best way to achieve this is to move toward a gold-backed currency, but his campaign pledge to tie the peso to the dollar would mark a step in the right direction.