ON THE battlefield Ukraine has more than once turned looming catastrophe into partial success against Russia. It might have pulled off a similar feat in the diplomatic realm on March 11th, when it agreed in principle to an American proposal for an “immediate” 30-day ceasefire. The commitment to stop fighting—if Russia reciprocates—was enough to unblock the flow of American weapons and intelligence. It may also turn the tables on the Kremlin. “We’ll take this offer now to the Russians, and we hope that they’ll say yes; that they’ll say yes to peace,” said Marco Rubio, America’s secretary of state. “The ball is now in their court.”