The United States and its most powerful adversaries are racing toward development of a new class of ultra-fast and deadly missiles that can evade modern defenses.

The Pentagon elevated hypersonic weapons, which fly faster than 3,100 mph, to a top priority over the past year as Russia and China move to field their own. But the military’s work might be put on hold due to a new order from President Trump to slash next year’s defense budget.

Hypersonics are among a slew of programs aimed at modernizing the military that could be on the chopping block as the Pentagon attempts to reduce next year’s total national defense budget from the planned $733 billion to $700 billion at the president’s direction.

“The exercise that we are going through is there’s prioritization that we can make. So for example … we have a number of options going on with hypersonic missiles and these projects we can choose either to do them or defer them,” Deputy Defense Secretary Pat Shanahan said on Friday.

The $700 billion represents total national defense spending, meaning the Defense Department budget as well as other expenses for the nuclear arsenal and law enforcement.

David Norquist, the DOD comptroller, is now combing through the $733 billion budget the Pentagon built over the past year looking for cuts, Shanahan told a military reporters conference.

“How fast do we modernize that is probably the biggest knob that we have to turn,” Shanahan said about where spending may be cut. “We will do as directed by the president and give him a $700 billion budget and then everybody gets to decide how to work with that.”

The development of new technology such as hypersonics and the purchase of new modern hardware is one of the easiest places for the Pentagon to cut when it has to. Those areas suffered under budget caps in recent years but began to get some relief as Trump oversaw hikes in defense spending over the past two years.

“It is the usual suspects of what can be modulated, because DOD has really considerable high fixed costs that they can’t change, like salaries or defense health program that is growing around 4 percent like every year. The places they usually can get money out fast are modernization and procurement, and operations as well,” said Frederico Bartels, a defense budget analyst at the Heritage Foundation.

Shanahan mentioned hypersonics as one example of potential cuts.

The Air Force signed contracts with defense giant Lockheed Martin worth $1.4 billion this year to develop missiles that can “travel more than five times faster than the speed of sound to overcome enemy defenses,” the company said at the time.

But defense modernization cuts could also encompass some of the Pentagon’s other flagship weapons initiatives, such as its most advanced fighter jets, a replacement for the Navy’s littoral combat ships, and attack submarines.

“Obviously we’re talking about moving away from some of the legacy systems and towards modern systems like the F-35, development of new programs, moving from the LCS to the frigate, as well as looking at the Columbia-class sub,” said Seamus Daniels, a defense budget analyst at the Center for Strategic and International Studies.

The Pentagon could choose to slow or delay the major acquisition programs by buying fewer aircraft or ships, but that could also increase the cost of those same programs down the road, said Travis Sharp, a research fellow at the Center for Strategic and Budgetary Assessments.

“Of course when you do that you generate inefficiencies because the contracts and the production is all structured around producing a certain quantity, and if you reduce the quantity there is going to be tradeoffs there,” Sharp said. “But of course if you’re being told at the political level to do that then that’s what you do.”

However, those hard choices all depend on Trump’s budget cut becoming a reality, which for now remains far from certain.

Shanahan said the Pentagon is holding onto its $733 billion defense budget proposal and Defense Secretary Jim Mattis will be using it to weigh the potential tradeoffs, which is an indication the military believes it has negotiating room with the president.

“We’ve been going through this very disciplined process for the whole year to build a budget that is $733 billion, and then last week we were directed, ‘Build us a $700 billion budget.’

"So we are not going to reverse course on all of that planning, but we will build two budgets,” Shanahan said.

The move is like throwing a political curveball into the budget debate between the Pentagon and the White House, and could give defense hawks in Congress a chance to exert pressure and increase spending on their own, Sharp said.

“I can anticipate that it is not going to be very difficult for people to get ahold of that [$733 billion] budget,” he said. “Even if DOD does not initiate that because that might not be considered responsible, Congress is going to want to find a way to get a copy of that bigger budget. Then Congress will be able to evaluate for itself what the tradeoffs were between $733 billion and $700 billion.”

The administration still has months to finalize its 2020 budget request, which is expected to be submitted to Congress in February, and as usual lawmakers will use it as a guide while putting together their own final defense budget.

“It’s all a political bargaining process and the number being $700 billion now doesn’t mean that that is what the number will be at the end,” Sharp said.