Some folks seem to think that when the annual defense budget lands at the Pentagon, the budget chief takes a pizza slicer and automatically splits it evenly among the three military departments. This year’s big defense budget debate has centered on whether the Army needs to give up some of its “share” of the funding pie to fund the Navy.

Given the rancor of this debate, you’d think that merely suggesting that one department might get more than another somehow violates a sacred practice. It doesn’t.

The Pentagon comptroller would have an incredibly easy job if all he had to do was just split the pot evenly. But it has never worked that way.

Sure, there have been a few years that all three department budgets were close percentage-wise. However, since the end of World War II, those years have been the exception rather than the rule.

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In the real world, the defense budget must respond to the current security environment and what the services need to fulfill their roles in the defense strategy. Those things shift over time, as you can tell by looking at the budgets of the military departments through time.

From the decade between 1954 and 1964, for instance, the Air Force was viewed as the service most responsible for America’s nuclear deterrence mission. Consequently, it received a bigger slice of the defense budget: 43%. In that same period, the Navy and Army averaged 29% and 26%, respectively.

In the decade following the Vietnam War, including most of the Reagan build-up, the Navy received the largest share of the budget. It averaged 33% of the defense dollars, partially because of the goal to get to 600 ships. The Air Force got 30% of the pie, and the Army had the smallest slice at 25%.

From 1990 to 2000 (post-Cold War, pre-9/11), the United States emphasized expeditionary forces to handle smaller conflicts and increase power projection. Again, the Navy received the biggest budgetary share, with an average of 31% of the defense dollars. The Air Force was second with 29%, and the Army lagged behind with 26%.

What about the missing 14%? This was the era which saw the rise of defense-wide accounts, which support the business functions of the entire department. Defense-wide, also known as “fourth estate” accounts, continued to grow in the next decade. By 2015, they consumed almost 18% of the defense budget. In the last five years, they’ve averaged around 17%. The fourth estate has become a very substantial part of the defense budget, further weakening the notion the defense budget is split equally among the three military departments.

In the 20 years after 9/11, the United States was engaged in land wars in the Middle East. Because of this, the Army started receiving the biggest budgetary slice, with an average of 30% with the Navy at 27% and the Air Force at 26%.

That trend is already changing, however. In the 2020 budget, the Army was back to getting the smallest share of the pie.

These fluctuations respond more to the current National Defense Strategy than the tired trope that each of the departments is entitled to one third of the defense budget. Each of the departments will require relatively more—or fewer—resources depending on what the nation is asking it to accomplish and what future the Department of Defense is preparing for.

If the country is engaged in, or anticipates a need for manpower-intensive land operations, it is very likely that the Army will receive a big share of defense dollars. By the same token, if the country is bent on beefing up its naval fleet, the Navy’s share will likely grow.

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The budget problem the Department of Defense now faces is that every service, from the Army to the one-year-old Space Force, is currently undergoing some overdue expansion or modernization, yet the administration has opted not to ask Congress for a real increase in the defense budget. And, it’s simply impossible to expand or modernize on less. It couldn’t be done even if the Pentagon comptroller possessed a magic pizza-cutter.

The budgets of individual military departments will naturally ebb and flow to match the national security needs of the country. By the same token, the overall defense budget needs to ebb and flow to meet the national security challenges.

The administration’s defense budget is, unfortunately, an ebb-tide budget, even though the United States finds itself facing increasingly assertive adversaries. The defense budget needs to rise to meet the urgency of the moment.