Trump regularly takes a hard-line tack when it comes to negotiations with U.S. allies, often floating more extreme scenarios while trying to nail down specific commitments. He floated the 4 percent figure while pushing allies to reach the current goal of spending 2 percent of their gross domestic product (GDP) on defense.

Experts aren't sure the U.S., which already spends more on defense than the next eight largest defense spenders combined, needs the extra boost, while budget watchers are concerned that substantially expanding defense spending would further inflate an already-mammoth deficit.

ADVERTISEMENT

“You should set the strategy first, based on security needs, and then let the budget flow from that,” said Todd Harrison, director of defense budget analysis at the Center for Strategic and International Studies (CSIS), a think tank in Washington, D.C.

“I think it’s a ridiculous metric to use. Why do you want to tie your level of defense spending to something that’s totally unrelated?” he added.

Currently, as part of an agreed-upon goal, NATO requires all 29 of its members to spend at least 2 percent of their GDP on their own defense by 2024. The alliance has said it expects eight members to hit the target this year, and 15 are on track to reach the goal by the deadline.

Trump has demanded that the countries move faster to hit the target, and suggested that the “delinquent” countries owe the United States for their shortfalls.

After floating the 4 percent figure at NATO, Trump on Thursday said that European nations had agreed to "substantially up their commitment" to defense spending, though he did not provide specifics on which countries made such an agreement or how much they would spend, saying only that he wanted countries to reach the current 2 percent goal "immediately" rather than gradually.

But Harrison argues that defense should be tied to security needs, not the size of a nation’s economy, and also cast doubt on the importance of questioning those who are able to spend less while providing sufficient troops and equipment.

Marc Goldwein, senior policy director for the Committee for a Responsible Federal Budget, a budget watchdog, agreed.

“It’s not obvious that if real GDP grows 20 percent in the next 10 years, that we need 20 percent more tanks and airplanes,” he said.

A recent CSIS report on NATO spending suggested that setting defense target relative to overall government spending would be a better measure.

“Assessing the financial contribution of a member state relative to its economy fails to take into consideration how the money is spent and what it adds to the collective defense effort. It similarly does not assess the efficiency with which it is spent,” the report said.

Historically, U.S. defense spending has accentuated that mentality.

During the Cold War and at the heights of the wars in Iraq in Afghanistan, the U.S. regularly spent above 4 percent of GDP on defense, according to Harrison.

In recent years, however, it’s been lower. Since 1994, the average U.S. defense expenditure amounted to 3.6 percent of GDP.

Beyond that, the military could have trouble finding good ways to spend all that extra cash.

“I don’t think it would necessarily be spent effectively, in ways that make us safer,” Harrison said, noting that the Pentagon generally gets the funding it requests to fulfill its strategic missions.

Then there’s the question of how the U.S. would pay for the higher defense costs.

In April, the nonpartisan Congressional Budget Office projected that 2018's defense outlays would amount to $622 billion, which it pegged at 3.1 percent of GDP. That would mean that in 2018 alone, the United States would need to spend $181 billion more to hit the GDP target, a 29 percent increase over current defense spending levels.

And that’s just the tip of the iceberg.

“That’s the one-year number, but the actual cost over a decade would be closer to $3 trillion,” said Goldwein.

“That’s the tax bill times two," he said, with higher interest payments pushing the cost even higher.

“I haven’t seen any kind of evidence that we need this kind of defense spending,” Goldwein added.

Some fiscal and defense hawks say that the issues of debt and security are linked.

Former Joint Chiefs of Staff Chairman Adm. Michael Mullen called the national debt the “most significant threat to our national security.”

One reason is that the more money is tied up in the military, the less flexibility the United States has to respond if a sudden new threat emerges. Another is that higher debt levels will make it harder to gather the funds needed to pay for defense in the future.

“The greatest threat to our nation is not North Korea, or Russia, or even [the Islamic State in Iraq and Syria]," he said last year. “The greatest threat to our nation is our inability to get our fiscal house in order.”