Republican presidential nominee Donald Trump’s US military buildup plan would cost hundreds of billions of dollars - but with no apparent strategy, defense experts from across the political spectrum said on Thursday.

“I haven’t seen any kind of strategy,” said William Hartung, director of the Arms and Security Project at the Center for International Policy. “He (Trump) says nobody is going to challenge us because we will be so strong. But that’s not a strategy. It’s just a kind of wish-fulfillment.”

US Senator Jeff Sessions, a top Trump backer who sits on the Senate Armed Services Committee, said the proposal was based on recommendations from groups such as the National Defense Panel and served as a statement of Trump's commitment to build the military.

“I believe this lays out a framework for rebuilding the military, and it represents a commitment by Donald Trump to make this a priority,” Sessions said in an interview. “If you don’t have presidential leadership really defending the need for a robust national defense, you're not going to maintain the defense budget.”

Trump's proposal, unveiled in a speech on Wednesday, did not spell out how he would accommodate the additional manpower and hardware as the United States shutters military bases, or where and for what purposes the larger forces would be employed. There were no cost estimates and Trump proposed revenue-raising steps that budget experts called insufficient.

“He just called for higher defense spending without giving us a number and without telling us how he is going to pay for it,” said Lawrence Korb, a former Reagan administration Pentagon official and senior fellow at the Center for American Progress, a think tank aligned with the Obama administration.

Trump’s Democratic opponent in the Nov. 8 election, Hillary Clinton, advocates tough defense and foreign policies, but has yet to take a stand on the size of the Pentagon budget.

Stephen Miller, a Trump policy adviser, said Trump's proposal came in contrast to Clinton, who he said has "no military plan."

Trump pledged to expand the Army to 540,000 active-duty troops from its current 480,000, increase the Marine Corps from 23 to 36 battalions – or as many as 10,000 more Marines – boost the Navy from 276 to 350 ships and submarines, and raise Air Force tactical aircraft from 1,100 to 1,200.

Trump said those numbers were based on assessments by the conservative Heritage Foundation and other groups. Heritage said in a report that it looked at the capacity needed to handle two major wars to determine its force-size recommendations.

Trump said he would bolster the development of missile defenses and cyber capabilities. He made no mention of US nuclear forces already in the midst of a modernization effort that will cost an estimated $1 trillion over 30 years.

To pay for the buildup, Trump said he would ask Congress to lift a Pentagon budget cap and “fully offset” the increased costs by collecting unpaid taxes, cutting appropriations for federal programs operating without congressional reauthorization, cracking down on social welfare fraud and other fraud, and collecting additional taxes and fees from increased energy production.

'SOFT-PEDALING' THE COST

Writing in The Weekly Standard, a conservative magazine, Tom Donnelly, a defense scholar at the American Enterprise Institute think tank who opposes Trump’s election, praised Trump for embracing a buildup that many mainstream Republicans advocate.

“However, Trump undercut the power of his proposals by soft-pedaling the cost of such a buildup,” he wrote.

Independent cost estimates for Trump's plan range from $150 billion in additional spending over 10 years, according to the bipartisan Committee for a Responsible Federal Budget, to as much as $900 billion over the same period, as assessed by Todd Harrison, a defense budget analyst at the Center for Strategic and International Studies think tank.

Harrison said that increase could be achieved only by raising the federal budget deficit, raising taxes, or cutting other spending, such as benefits programs for seniors and the poor. “None of those things are politically popular,” he said.

The Committee for a Responsible Federal Budget estimated that lifting the cap would cost $450 billion over 10 years. The revenue-generating steps proposed by Trump would leave $150 billion of that amount uncovered, it said.

Another flaw in Trump's plan is the assumption that Republican members of the House of Representatives who belong to the deficit-fighting tea party movement would agree to end the budget cap.

In April, Army Chief of Staff General Mark Milley told a Senate committee that adding more soldiers without a sufficient budget would be disastrous for the country and the Army. Bases would close and programs that support troops and their families would have to be curtailed to make up the shortfall, he said.

The Navy already has launched a shipbuilding program to raise the number of vessels to more than 300 by 2021. Trump's plan fails to account for the country's limited shipbuilding capacity and the cost of manning, maintaining and basing the additional warships he proposes to build.

“The whole thing is unrealistic,” said Dov Zakheim, the Pentagon’s top financial official under former President George W. Bush. Zakheim, who opposes a Trump presidency, estimates that Trump’s plan would boost defense spending by roughly $300 billion over five years. “It’s a soundbite,” he said.