Recent shipments of U.S. and allied weapons, particularly the Himars multiple-launch rocket systems and 155mm howitzers, have helped to blunt Russia’s offensive in the Donbas and stabilize the situation there, Mr. Zelensky said. The Russians used to fire 12,000 artillery shells daily against 1,000 to 2,000 by Ukraine, he said. Now, he added, Ukraine can fire some 6,000 shells a day while Russia is beginning to feel a shortage of ammunition and troops. This change in the balance of firepower has stemmed Ukrainian casualties, Mr. Zelensky said. At the peak of fighting in May and June, he said, Ukraine was...