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.

The war in Ukraine is an artillery-centric conflict because both Russia and Ukraine — thanks to their shared Soviet inheritance — have artillery-centric militaries. If Kyiv can’t find enough artillery pieces and ammunition, especially 155mm shells, it will be at a dire firepower deficit along the conflict’s front lines. It could face a replay of the situation it faced in mid-2022, when its outgunned forces were being pummeled in the east.

Likewise, if Ukraine can’t find sufficient air defenses, it can’t protect its people or its forces from Russian missile, drone and aircraft attacks. And if one reason Russian President Vladimir Putin thinks he can win this war is the prospect of a change in policy after next year’s US presidential election, another is that he believes he can outlast Ukraine and its backers in the areas that matter most.

This is the key to understanding Putin’s persistent air campaign. Strikes by Russian missiles and drones aren’t meant solely to kill Ukrainians and ravage infrastructure; they are also meant to exhaust Ukraine’s air defenses, so Russia’s planes and helicopters can establish superiority in the air and shape events on the ground. F-16s and other advanced fighters could eventually reduce that Russian air superiority, but it will take time to get Ukrainian pilots up to speed.

Meanwhile, the US and other countries spent this spring scrambling to help Ukraine hold together an air-defense network using everything from Patriot missiles to Soviet-era systems. Even so, a lack of short-range air defenses is allowing Russian combat aviation to take a toll on Ukrainian forces assaulting the front lines.

The artillery shortfall is even more daunting. US officials estimate that Putin can produce perhaps 1 million rounds of 152mm artillery ammunition per year as Russian industry gears up. The US produces one-seventh as much, although it hopes to double the number by year’s end and raise it sixfold over a half-decade. European production figures are less impressive still.

This creates a window of danger lasting at least through late 2024. If Putin can hang on through the current Ukrainian offensive and all the turmoil in Russia, perhaps he can indeed put Ukraine under the gun again.

That possibility is driving a desperate global scavenger hunt for munitions. Russia is buying artillery, drones and missiles from Iran and North Korea. The US has reportedly pried loose thousands of 155mm rounds from South Korea; it is trying something similar with Japan; it is “repositioning” rounds stored in Israel to Ukraine. New North Atlantic Treaty Organization member Finland, with its formidable artillery forces, is dispatching howitzers and rocket launchers to Ukraine. American officials are scouring Peru, Egypt and other locales for vital weapons.

The artillery shortfall is also leading the US to mull a distasteful option. Last week, Pentagon officials told Congress that Washington possesses one capability in sufficient numbers to make a difference: cluster munitions. Delivered by artillery or surface-to-surface missiles, these munitions contain small bomblets that saturate the target area, devastating armor and people. Giving them to Kyiv could create a legal and diplomatic mess.

Cluster munitions are a humanitarian nightmare, not simply for the carnage they wreak when deployed but for the unexploded, lethal bomblets they leave behind. More than 100 countries, including most American allies (but not the US itself), have signed an international treaty banning them. Opposition to using cluster munitions in Ukraine is also significant on Capitol Hill.

Yet Ukrainian President Volodymyr Zelensky made a strong push for cluster munitions at the Group of Seven summit in May. And if moral concerns about these weapons are real, they must be balanced against the costs — moral and strategic — of denying Ukraine a means of survival in a war it did not choose.

If the fighting drags on beyond this summer, don’t be shocked if cluster munitions eventually fall in the same category as Abrams tanks, HIMARs, and F-16s — other weapons the US swore it wouldn’t provide to Ukraine up until the moment it did. In a war where the geopolitical stakes are severe, and the prospects of Ukrainian success uncertain, there are no easy choices left.

More From Bloomberg Opinion:

• And Now Putin Decides Which Bugs to Crush: Andreas Kluth

• Russia’s Uprising Is a Serious Threat to China as Well: Hal Brands

• Wagner Mutiny Foreshadows a Russian Defeat: Leonid Bershidsky

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Hal Brands is a Bloomberg Opinion columnist. The Henry Kissinger Distinguished Professor at Johns Hopkins University’s School of Advanced International Studies and a senior fellow at the American Enterprise Institute, he is co-author of “Danger Zone: The Coming Conflict with China” and a member of the State Department’s Foreign Affairs Policy Board. He is a senior adviser to Macro Advisory Partners.

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