WASHINGTON — As the White House prepares to ask Congress for steep cuts to the U.S. State Department and United States Agency for International Development, the military's top commanders are telling lawmakers that slashing diplomacy would undercut the military.

Over the last two months, Defense Secretary Jim Mattis, Joint Chiefs Chairman Gen. Joseph Dunford and six combatant commanders — most recently U.S. Special Operations commander Gen. Raymond Thomas — have  given fodder to lawmakers who argue that U.S. foreign policy cannot thrive on military might alone and that a full State Department budget is vital to national security.

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In an exchange with Sen. Elizabeth Warren, D-Mass., during his Senate Armed Services Committee appearance May 4, Thomas said cuts to diplomacy and foreign aid "would make our job harder." The State Department is "indescribably critical" to U.S. Special Operations Command's mission to advise and assist local forces to build their own capacity, he said.

"We are in 80 different countries, and we look to have the most enhanced relationship possible with every one of those countries through our country team," Thomas said. "If that's not the baseline for our United States government approach, then we are flawed from the start."

Top brass 'getting even louder'

The testimony comes amid news Secretary of State Rex Tillerson is probing his agency for waste, redundancies and inefficiencies — with a survey of its 75,000 employees. The White House's 2018 budget proposal, expected to land on Capitol Hill in late May, is expected to call for a 28 percent cut to the State Department and USAID.

As the White House and congressional Republicans seek to curb the nation's deficit, those cuts would offset the White House's proposed $54 billion increase in defense spending.

But the military seems to be pushing against the idea of depleting the State Department. Mattis — who asserted in 2013: "If you don't fund the State Department fully, then I need to buy more ammunition ultimately" — said in Senate testimony on March 22 that national security is "a critical team effort, and we intend to keep it that way."

"America has two fundamental powers: the power of intimidation and the power of inspiration. Soft power is largely found in the power of inspiration and is part and parcel of how we defeat this enemy,"  Mattis said of the Islamic State group.

More than 120 retired generals and admirals, including former CIA Director David Petraeus, signed a letter last month organized by the U.S. Global Leadership Coalition that urged Congress not to slash funding for diplomacy and foreign aid.

"The volume of top brass is getting even louder around this issue," U.S. Global Leadership Coalition CEO Liz Schrayer told Defense News. "This is a voice of power speaking out, that civilian forces are complementary to the military. … We need the work of State and USAID."

Speaking on May 5 — the day news broke that a U.S. service member was killed during a counterterrorism operation in Somalia that targeted the militant group al-Shabab — Schrayer said the prospect of rolling back aid in Africa worries commanders there. Somalia is one of several African countries approaching famine. 

U.S. Africa Command's Gen. Thomas Waldhauser told the Senate Armed Services Committee on March 9 that the command's partnership with the State Department and USAID was "key to achieve enduring success."

"Together, we work to address the root causes of violent extremism, lack of accountable government systems, poor education opportunities, and social and economic deficiencies to achieve long-term, sustainable impact in Africa," Waldhauser said.

As ISIS nears collapse and U.S.-backed forces seem poised to liberate the militant group's strongholds in Mosul, Iraq, and in Raqqa, Syria. Anthony Cordesman, of the Center for Strategic and International Studies, asked who would stave off the humanitarian crises expected to follow, if not the State Department and USAID.

Cordesman offered that civil aid is critical to improving governance, reducing corruption and stabilizing economies following a military victory — "or you wind up, as we did in Iraq, watching the structure of governance collapse, and a group like ISIS sweeps in."

"After a military defeat of an enemy, you have to have some form of recovery and humanitarian plan, or all of the ethnic tensions that led to civil war and the rise of ISIS and al-Qaida in the first place are going to come back, just as they did in Iraq," Cordesman said.

"The problem we have is we're cutting aid, and what's more important is we have no plan as to how we deal with stability and humanitarian issues (in Middle East conflicts)," Cordesman said. "Instead of dealing with a strategic requirement, you're making cuts that will take already weak aid programs and cripple them."

Brett Schaefer, a foreign policy expert with the conservative Heritage Foundation whose  2016 paper is reportedly influencing budget plans, said the impact of the cuts has been blown out of proportion. The idea, he said, is to reverse Obama-era growth and return to budget levels for the State Department seen under the Bush administration, "which at the time was engaged very robustly abroad."

Advocating the cuts be phased in over several years — as opposed to the administration's all-at-once approach — Schaefer said bureaucratic streamlining could target the Obama-era State Department's proliferation of "non-traditional" bureaus, czars, advisers and envoys focused on issues that seem to be less of a priority for the Trump administration, like climate change and human rights.

"You can actually do these reductions and not impact the core diplomatic functions and engagement of the State Department by addressing the cuts at the areas where we've seen the most expansion," Schaefer said. "People are reacting to the size of the cuts and are not looking at it from the perspective of how it has been historically."

The power of the purse

The White House may have proposed slashing the State Department budget, but some key lawmakers, including powerful Senate Majority Leader Mitch McConnell, R-Ky., have already balked at the idea. 

In April, Sens. Richard Durbin, D-Ill., and Todd Young, R-Ind., led a bipartisan group of more than three dozen senators urging "robust funding" for the fiscal 2018 international affairs budget. Durbin is the top Democrat on the Senate Appropriations Defense Subcommittee.

"At a time when we face multiple national security challenges around the world, deep cuts in this area would be shortsighted, counterproductive, and even dangerous," read the letter from 43 senators to the respective chairmen and ranking members of the Senate Budget and Appropriations committees.

Sen. Lindsey Graham, R-S.C., chairman of the Senate Appropriations Subcommittee on State, Foreign Operations, and Related Programs,  told an audience at the Freedom House annual awards ceremony in D.C. in late April that he was a "pretty hawkish guy, but I understand the limits of military power."

"We're cutting the State Department's budget by 29 percent and, over my dead body, that ain't happening," Graham said.

"I have learned a lot going to Iraq and Afghanistan that this war will never be won by military force alone," he said. "You can't kill enough of these bastards to destroy radical Islam. What you have to do is build up the lives of others who reject the ideology. They just need your help. They're offering a glorious death. We need to offer a hopeful life."

Senate Foreign Relations Committee Chairman Bob Corker, R-Tenn., and Ranking Member Ben Cardin, D-Md., said separately that the deep cuts proposed by the White House were unlikely to prevail, but they were open to the ongoing reassessment of U.S. priorities for diplomacy and foreign aid. 

"We spend 1 percent of our budget on diplomacy and aid, and every military general will tell you if you spend less — we always have to make sure it's effective and there's always things we need to do to be fine-tuning, re-looking, making sure it's delivering the results we expect," Corker told Defense News.

"I think there's a strong support in the United States Senate and the House of Representatives for trying to use diplomacy to keep our men and women in uniform from being in conflict, and I think that's going to continue," Corker said.

Cardin said Trump's aid plans should be tied to its foreign policies, "which haven't been articulated yet," adding: "Maybe the first articulation will be in the budget." 

There's an urgent need for more aid, not less, for Democracy-building in Africa and for the aftermath if ISIS is ousted from Mosul and Raqqa, Cardin said.

"Let's do things more efficiency, but we will need a strong enough budget to meet urgent needs out there that are more than the current budget, greater than the current need," he said. "I hope here is no common direction between the Trump budget and where we end up."

Correction: A previous version of this story misidentified the state that Sen. Bob Corker serves. This updated version reflects that he is a senator for Tennessee .

Joe Gould was the senior Pentagon reporter for Defense News, covering the intersection of national security policy, politics and the defense industry. He had previously served as Congress reporter.