WASHINGTON — When the two-year federal budget pact announced earlier this month hewed fairly closely to the president's requested defense budget, defense hawks and industry leaders applauded. The bipartisan deal upended pessimistic predictions of a year-long continuing resolution that would have kept defense spending flat and prohibited new-start programs.

But experts warn that beyond the pact's horizon, the picture gets fuzzier, subject to the forces of national politics, inter-service rivalries, global politics, and the strength of the US economy. The 2011 budget caps were eased for 2016 and 2017, but not repealed, and the US faces major national security challenges in the Mideast, Europe and the Pacific.

Derek Chollet, a former assistant secretary of defense for international security affairs who is now with the German Marshall Fund, said the US must find a way to juggle and sustain those three regional efforts, adding . He said it is hard to see how without a growing defense budget.

"What's been argued is that the current administration doesn't have its priorities accurately reflected and hasn't created enough resources to meet these challenges. But the next president is going to have to confront all of those challenges on their plate at the same time," Chollet said. "And he will have to figure out what to do about it — and how to pay for it."

Experts speaking Monday at the Global Security Forum, sponsored by the Center for Strategic and International Studies (CSIS), painted differing pictures of the prospects of future defense budgets.

Several said it seemed likely that the use of wartime overseas contingency operations accounts to fund base budget requirements, as a way of skirting budget caps, will likely continue in future years as a way of skirting budget caps.

"It's not an optimal way to budget, but people just need to realize this is the product of a Congress that has been fighting amongst itself for a number of years," said Senate Armed Services Committee Staff Director Christian Brose. "Let me say that shortfalls are probably worse."

In the Republican presidential debate, Sen. Marco Rubio and Sen. Rand Paul sparred over whether robust defense spending is equatable with conservativism. Paul, a Kentucky libertarian and non-interventionist, chided Rubio, of the Florida senator, over his plans to increase the defense budget by $1 trillion over the next decade without explaining how to pay for the hike.

"How is it conservative to add a trillion dollars in military expenditures?" Paul asked. "You cannot be a conservative if you're going to keep promoting new programs that you're not going to pay for."

Rubio replied: "We can't even have an economy if we're not safe."

Similar arguments over the Pentagon’s top-line will play out among the candidate and in Congress, Brose predicted. Former Undersecretary of Defense (Comptroller) Tina Jonas, now with CSIS, added that defense hawks will continue to cite arguments the lack of budget stability and readiness cuts of previous years, as well as the national security threats of the moment — yet Jonas agreed, but said she didn’t expect much change from the budget deal.

"It's going to be a political year, things will be exciting," she said, "but my prediction is that we will come up somewhere near the budget deal."

Beyond 2016, the budget is expected to experience pressure related to modernization and procurement, absent a "grand bargain." Steve Kosiak, a former Office of Management and Budget defense expert, predicted "relative stability" in procurement and —  unless there is a major upswing in funding — tension between that, force structure and healthcare costs.

While Kosiak was happy with the deal, he said defense watchers should not presume there will be a good deal every two years. "I wouldn't paint too rosy of a picture going forward," he said.

Programs such as the Ohio-class ballistic missile submarine replacement, the Air Force's new bomber and tanker, and the tri-service F-35 joint strike fighter will have to compete for resources.

The next administration could spell opportunity for a new tranche of funding, as candidates are already grappling with whether the current path for national security is adequate.

"My sense [is] you will have candidates in both parties arguing, not for a once-in-a-generation buildup, but a buildup beyond what the program is currently," Brose said, adding later: "As you look to next year's election, spending less on defense doesn't strike me as a winner."

Yet how such a buildup will be funded in the federal budget remains an open question, Kosiak said. Will there be a tax hike or tax cuts? Will entitlements be cut or left alone? Will there be parity on the non defense side?

"Both Republicans and Democrats agree we need more money for defense," Kosiak, "but how does that fit into the overall [federal budget] package."

Mackenzie Eaglen, an American Enterprise Institute analyst and former congressional defense aide, said Congress will have to find funding for the Pentagon's efforts in the Mideast, Europe and the Pacific, or it will have to choose from among some politically unpopular options: shuttering units, closing bases, or cutting contractors or depot workers.

"There's no magic sauce here," Eaglen said. "The entire economy needs to grow or you're cutting defense under any scenario."

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