The U.S. says there will be a "firm response" as Ukrainian forces holding ground in Russia's Kursk region now face North Korean troops "engaging in combat."

Russia needs foreign troops if it expects to keep up its war effort in Ukraine through next year. It needs foreign equipment, too. As the wider war on Ukraine grinds toward its fourth year, the stocks of old Cold War weaponry the Kremlin depends on to equip its regiments are running dangerously low.