The United States possesses the world’s most advanced military equipment, and quality matters immensely in combat. But quantity gets a say, too. And from ships to shells to soldiers, the U.S. military lacks the personnel and materiel it needs to fight a major war.

America’s armed forces, with a naval fleet roughly half the size it was in 1987 alongside an increasingly smaller and older fleet of combat aircraft, are equipped only for short, sharp, high-intensity conflicts.What happens when a war is longer and more violent? Ukraine’s fight against Russia, Israel’s battles in the Middle East and recent U.S. operations against the Houthis in Yemen offer a preview of the demands of modern war and demonstrate why America requires more than we have now to win a large conflict.

Defense Secretary Pete Hegseth’s mandate from President Trump to refocus the Pentagon should ensure America’s military has the resources to endure and win a large-scale war. Progressives and fiscal hawks have their knives out for military spending, but the secretary should refrain from cuts to resources that directly strengthen America’s combat power, including active service members, ammunition, new ships and new aircraft.

There is no magic number of dollars that the United States should be spending on defense, but the resources we have now are not enough to meet the simultaneous challenges posed by China, Russia, Iran, North Korea and terror groups across the globe. The further our defense capabilities fall behind the risks we face, the more costly it will be to catch up.

How did this happen? Over the past few decades Washington has traded a larger, more durable military force for one that is high-tech but brittle. Defense planners have made small quantities of expensive, cutting-edge weapons a priority over producing enough firepower for a long fight.This shift relied on advanced weapons having the power to end fighting decisively and quickly. Yet modern wars have evolved to require both advanced systems and sheer volume of 20th-century staple munitions, such as artillery shells and missiles.

Ukraine is using up to 15,000 artillery shells a day in its fight against Russia’s invasion. The United States produces only 40,000 shells each month.