President Biden warned China two years ago not to provide “material support” for Russia’s war in Ukraine. On Friday, Secretary of State Antony Blinken conceded that Xi Jinping ignored that warning. China, Mr. Blinken said, was “overwhelmingly the No. 1 supplier” of Russia’s military industrial base, with the “material effect” of having fundamentally changed the course of the war. Whatever Mr. Biden chooses to do next will be momentous for global security and stability.

Mr. Biden can either enforce his red line through sanctions or other means, or he can signal a collapse of American resolve by applying merely symbolic penalties. Beijing and its strategic partners in Moscow, Tehran, Pyongyang and Caracas would surely interpret half-hearted enforcement as a green light to deepen their campaign of global chaos. Mr. Xi sees a historic opportunity here to undermine the West.