For a moment, Ukraine had caught the break it needed and deserved.

Mercenary chief Yevgeny Prigozhin’s forces were advancing on Moscow; Russia was facing a civil war that would have crippled its war of aggression in Ukraine. We don’t yet know what the fallout from the recent short-lived revolt will be: Mutiny is rarely good for the morale, cohesion or effectiveness of the troops in the trenches. But if a crisis that ended as startlingly as it began doesn’t devastate Russian defenses, Kyiv and Washington may soon find themselves returning to the grim math of the war in Ukraine.

For months, debate in Western capitals has focused on whether to give Ukraine more sophisticated weapons — high-end tactical missile systems called ATACMS, Storm Shadow cruise missiles, F-16 fighter jets and more. These weapons would certainly help, but success in this war still hinges on abundance of two less sexy but more critical armaments: artillery ammunition and air-defense systems. If Ukraine can’t win this war quickly, through a Russian collapse or a heroic breakthrough on the battlefield, it could still be in danger of losing if it can’t replenish these arsenals.