Most Americans know the U.S. military is the most powerful in the world, and it has been for decades. This fact, however, gives false ideas about what our military is actually able to do today.

For example, one might assume the military has massive stocks of artillery shells, bombs, and other munitions that would last through the duration of any war the U.S. may find itself fighting.

But the war in Ukraine has demonstrated that this is not the case.

The U.S. began sending military aid to Ukraine early last February. By April, our stocks of Javelin anti-tank missiles and Stinger anti-aircraft missiles had been depleted by a third.

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If only two months of a regional war consumed that large a chunk from our critical munitions stockpiles, it is easy to imagine the military would run out of these munitions if the U.S. ever faced down a competitor like China.

The Army, Air Force, Navy and Marine Corps each has a sophisticated planning process to determine how much of each kind of munition it should keep on hand in case of war. The services base their plans on the Pentagon’s estimations of what kinds of wars, with which countries, are most likely to occur.

For years, however, the military services have budgeted less for munitions so they could budget more for their “big-ticket items” like tanks, fighter jets, and ships. Their stockpiles of munitions would last for only a few months of war.

But wars are unlikely to last only a few months, especially if the two countries are of comparable military strength. Just ask Vladimir Putin, who expected to overrun Ukraine in a matter of weeks.

So why not just make more munitions, replacing those that have been drawn down and then some? That’s the second part of the problem.

The defense industry will not be able to replenish some of these rundown stocks for years.

Take the Stinger missile as an example. Raytheon, the prime contractor for Stinger missiles, had ceased Stinger production before the Russian invasion. Raytheon CEO Greg Hayes says the company could not ramp up production for at least 18 months because the company had to redesign some of the electronics in the missile and the seeker head.

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Other munitions would have similar production timelines, for a variety of reasons. Ramping up production requires investment in the workforce, facilities and capital equipment. Suppliers of components and raw materials also have to invest in their workforce and facilities, and so on down the supply chain. A supply chain is only as strong as its weakest link, and a bottleneck for just one component—like a microchip in a seeker head—will cause delays for the entire program.

The math is simple. If the military has enough munitions for only a few months of war, and the defense industry requires 18 months to supply more, the U.S. will be in trouble if it has to fight a protracted war.

Congress funded the purchase of more munitions in the recent omnibus spending bill. It also increased its oversight of the military services’ munitions requirement planning processes, requiring the Pentagon to report on how the military makes its munition-stockpiling decisions.

This issue will require continued attention, however. Small stockpiles, and an industrial base that cannot quickly respond to increased demand, create a major vulnerability for U.S. national defense. Congressional and Pentagon leaders need to prioritize munitions. U.S. military preparedness depends on it.