The U.S. military is in rough shape. Only one-third of the Army brigades are combat ready. The Air Force is cannibalizing some F-16s to keep others flying. The Marines' inventory of spare parts is so low, they're filching parts from planes in museums. And the Navy keeps extending deployments because it has too few ships.

Congress, to its credit, seems to be paying attention. The House of Representatives recently passed a defense policy bill that would boost spending by $18 billion to start rebuilding the military. The Senate is now taking up its version, and Sen. John McCain, R-Ariz., plans to offer an amendment that would add $17 billion to do things like increase maintenance and training, grow the size of the Army, and buy more ships and airplanes.

Not everyone's on board. Defense Secretary Ash Carter and his senior team have attacked lawmakers' efforts to increase his budget. But Carter's arguments just don't add up. Let's look at each one in detail.

'Higher defense spending threatens budget stability'

Actually, the defense budget has been highly unstable in recent years. The Budget Control Act of 2011 led to a 25 percent real cut in defense spending, as well as two adjustments by Congress to prevent further cuts.

Carter's desire for budget stability is understandable, but this argument misses the main point.

First, a "stable" defense budget that leads to a weaker, smaller military is not a good budget. Budget stability is no fountain of youth that will magically restore our military. It won't add troops or equipment or buy more flying hours for Marine Corps pilots. Budget stability may allow for better management of resources, but the fundamental problem is a lack of resources. Rebuilding the military requires reinvestment.

Moreover, congressional efforts to boost defense spending won't lead to the same kind of budget instability we've seen in recent years. The debate is not about whether to reimpose devastating across-the-board cuts. It's about where — and how much — additional funding can be found to address increasingly apparent shortcomings in training, maintenance and force structure.

Uniformed and civilian leaders from across the Pentagon have made the case that our military is deteriorating due to underfunding. Does Carter really believe that spending more to address some of these problems is a bad move?

'Higher defense spending would unravel the Bipartisan Budget Act'

A parallel argument from Carter warns that a boost in defense funding could unravel the Bipartisan Budget Act, or BBA. Hammered out last November by the administration and then-House Speaker John Boehner, the BBA approved matching increases in defense spending and non-defense spending for fiscal years 2016 and 2017, thus producing the "budget stability" touted by Carter.

But the defense spending level agreed to in the BBA should be viewed as the floor, not the ceiling. And since Carter has been clear about the need for more money for defense and the need to unravel Budget Control Act spending levels in future years, why not start now? As Carter told Congress, "going forward, the biggest budget priority for us strategically is Congress averting the return of sequestration to prevent what would be $100 billion in looming automatic cuts."

So Carter wants to unravel one budget deal (the 2011 BCA) but not another (the 2015 BBA), and he wants at least $100 billion more for defense in future years, but not another dime in fiscal 2017. Fortunately, Congress seems to be seeing through this smoke and is focused on the simple bottom line: the military has an increasingly urgent need for more funding.

'Don't buy force structure now without a commitment to keep it fully funded in the future'

Carter criticized the House defense authorization bill, arguing that it was buying force structure without the resources to pay for it in future years.

It's true that Congress budgets one year at a time, and that may not be an ideal process, but it's silly to criticize a bill for not doing something that it can't do. Congress can't pay for the future costs of a larger Army up front, but it can make the strategic decision to grow the Army now, regardless of how it provides that funding.

Second, military leaders have been pretty clear that they need a beefed-up force structure to deal with current threats, not just theoretical threats waiting in the future. The Marines want to grow, not shrink. The Army ranks are at historic lows, and their leaders, including the new secretary, say they need more combat power. Navy fleet size is far short of current goal, and Navy brass are likely to announce this summer that even the current goal is inadequate. Across the board, the message is clear: today's military is too small given the threats facing America.

'Another continuing resolution is bad for DoD'

Carter and other senior Pentagon leaders regularly argue that not having a budget in place at the beginning of the fiscal year hurts the military. They are probably right, but that doesn't mean that Congress should just accept current inadequate funding levels.

Regardless of the debate about total defense spending, the sad news is that DoD will almost certainly not have a budget in place by Oct. 1 even if Congress strives for that outcome. President Obama has signed into law temporary defense funding bills (aka, continuing resolutions) every single year he has been in office. George W. Bush did the same in four of his eight years in the Oval Office. Regardless of party in the White House or in Congress, regardless of whether the defense budget is going up or down, the defense budget is unlikely to be signed into law before the start of the fiscal year.

The bottom line

If senior military leaders are to be believed, the U.S. military is becoming smaller and weaker even as serious threats are growing. Carter's boss may require him to oppose congressional efforts to increase the defense budget, but his arguments are pretty hollow. Instead of backing down, Congress should step up this year and increase defense funding so that the military can start rebuilding.