WASHINGTON ― Congress will receive President Donald Trump’s FY21 budget proposal for the Pentagon and other federal agencies on Monday, but don’t expect it to pass as drafted.

Now in the second year of a two-year budget deal, the FY21 defense budget’s top line is locked in $740.5 billion, which represents a 3 percent decrease from FY20, when adjusted for inflation.

Between Monday and it’s expected passage, there’ll be pressure from Congress, as usual, to change significant details, according to Center for a Strategic and International Studies defense budget experts Seamus Daniels, Todd Harrison, Mark Cancian and Wes Rumbaugh, who held a talk on Friday.

Here’s what to watch.

Fourth Estate

Defense Secretary Mark Esper announced he intends to move $5.7 billion from the defense agencies that make up "the Fourth Estate”―parts of the Defense Department that are not military services―for more “important priorities.”

That is to say priorities connected to the National Defense Strategy’s focus on competition with Russia and China. That will include research into hypersonic weapons, artificial intelligence and big data, fifth-generation communications technologies, nuclear enterprise modernization, space, missile defense and response force readiness.

In 2018, When Rep. Mac Thornberry, now the top Republican on the House Armed Services Committee was chairman, he had a similar idea and sought legislation to slash back-office support services. But Maryland Democratic Rep. Anthony Brown led a successful bipartisan effort to weaken that proposal, and in the end, Thornberry agreed to a less prescriptive plan for cuts.

Last week, spokesmen for Brown and Thornberry said they needed more details from the Pentagon.

“Congressman Thornberry is encouraged by Secretary Esper’s review effort,” Thornberry spokesman Claude Chafin said in an email. “It is clear that the requirements of the National Defense Strategy will require increased agility, new ways of thinking, and repurposed resources.”

“A bipartisan group of lawmakers in 2018 came together to reaffirm the importance of ‘Fourth Estate’ defense agencies as force multipliers and as important support for our war fighters," said Brown spokesman Christian Unkenholz. "The public servants at these agencies are critical to executing the National Defense Strategy and keeping this country safe. Their work allows commanders to focus on accomplishing their missions ... Bottom line - we shouldn’t risk capability and civilian jobs for marginal cost-savings.”

Slashing aircraft

Retirements for significant numbers RQ-4 Global Hawk drones, older F-15s, F-16s, B-1 bombers will be proposed in the Pentagon’s five-year defense plans―to pave the way for more copies of the F-15EX, F-35 and B-21 stealth fighter, Foreign Policy reported last week.

Harrison noted how Air Force plans to retire the A-10 showed just how fiercely lawmakers can fight when aircraft housed in their districts are threatened, suggesting they will almost assuredly push back here.

“The Air Force force reductions will be a violent fight,” Harrison said. “Regardless of where they actually come out, you’re talking about retiring planes that are in someone’s district.”

On the operational side, the Global Hawk’s intelligence, surveillance and reconnaissance capabilities make it one of the most used aircraft in the military. “The argument you’re going to run into with the F-15C’s is you’re retiring them sooner than we have replacements. I think Congress will want to see what the replacement will be because they’ll be nervous about having a shortfall in fighters,” Harrison said.

Nuclear weapons vs. Virginia-class submarine?

After the National Nuclear Security Administration won a battle to increase its proposed budget, the Navy was forced to drop plans for a second Virginia-class submarine in its proposed budget, Bloomberg reported last week―a move that has fueled speculation the submarine had been used as a “pay-for.”

Because DoD and NNSA, the federal agency that maintains America’s nuclear arsenal, are under the same budget cap for defense, the NNSA increase would have to come out of DoD in one way or another. Whether or not there was a direct swap within the administration, it’s an open question whether Congress will agree to either.

For FY20, NNSA won only a modest increase as Democrats sought a steep cut. Senate Armed Services Committee Chairman Jim Inhofe, R-Okla., is among Republicans like Thornberry and Senate Foreign Relations Committee Chairman Jim Risch, who support the proposed 20 percent boost for FY21, as Inhofe argues it’s needed to compete with Russian and Chinese development efforts.

Meanwhile, the Navy in December awarded a $22.2 billion contract to General Dynamics for the construction of nine Virginia-class submarines. General Dynamics Electric Boat Corp., based in Groton, Connecticut, with facilities in nearby Quonset Point, R.I., as the main contractor and Huntington Ingalls Industries based in Newport News, Virginia, as the main subcontractor.

Among the advocates for the Virginia-class with local ties to the program are SASC ranking member Jack Reed, D-R.I., along with House Seapower Subcommittee Chairman Joe Courtney, D-Conn., and ranking member Rob Wittman, R-Va. Courtney, for one, has argued that a two-per-year build rate for attack subs bests support the nation’s security and industrial base.

Missile Defense

How much will the administration seek to invest in the newly-announced Next-Generation Interceptor, the agency’s single biggest new start program and a follow on to the troubled Redesigned Kill Vehicle program, which the Pentagon cancelled in August? Both were intended to replace today’s ground-based interceptors.

Because the NGI is described as a ten-year program, “that’s gotten a lot of consternation about the plan and viability of that. That’s probably the single biggest thing to look for there,” Karako said.

It’s unclear whether a test of a SM-32a against an ICBM planned for early this year will spark plans to make that SM-32a an “underlay” to protect the continental United States. Karako questioned whether it would be accompanied by the necessary ground infrastructure and sensors or whether it would simply lead to the purchase of more missiles that might be placed in Poland, Romania or Aegis Ashore in Guam.

Will development of a space-based sensor layer capable of detecting and tracking hypersonic weapons remain within the purview of the Missile Defense Agency or migrate to another entity within the Pentagon’s research and engineering directorate, led by Michael Griffin?

The Pentagon’s FY20 budget proposed moving it to the Space Development Agency, but Congress rejected that out of concern it would be lost amid SDA’s larger mission.