ANALYSIS — Rather than stay out of the fray, the Defense Department has taken sides in a bitter and historic partisan brawl, choosing to fortify President Donald Trump’s stonewall rather than cooperate with Congress.

The Pentagon is a key player in House Democrats’ impeachment inquiry into Trump’s dealings with Ukraine. And defense officials, who have until now been largely unscathed by the investigation, have rejected congressional demands for information about their role in the White House decision to withhold $250 million in military aid to Ukraine.

Regardless of which party controls Congress or the White House, congressional oversight of the Pentagon is a crucial check on the military’s expansive power and its pocketbook. Congress invests north of $700 billion of taxpayer money in national defense every year, and both have an obligation to monitor exactly how it is spent.

The Defense Department’s inspector general has not yet acquiesced to a request from seven Senate Democrats — including top defense appropriator Richard J. Durbin of Illinois and Armed Services ranking member Jack Reed of Rhode Island — to launch an internal investigation into what happened with Ukrainian aid, which has totaled $1.5 billion since 2014.

And now, as we learned in an Oct. 15 letter from Robert Hood, assistant secretary of Defense for legislative affairs, the Pentagon is refusing to comply with a subpoena for documents related to the delay in the dispersal of Ukrainian aid.

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Congress tends to take offense when the executive branch doesn’t respect its oversight authority, says Mieke Eoyang, a former staffer on the House Armed Services and Intelligence committees who now heads the national security program at the left-leaning think tank Third Way.

“This is a political response from a political appointee,” Eoyang tells me, noting the Oct. 15 letter was not signed by Pentagon General Counsel Paul C. Ney Jr. “I would be surprised if this letter reflects the legal judgment of the department’s legal counsel.”

The Pentagon knew that information around the Ukrainian aid program would be of particular interest to Congress. That’s why Ney issued a memo Oct. 3 instructing staff to preserve any documents relevant to the Ukraine Security Aid Initiative and submit them to his office.

Pentagon chief spokesman Jonathan Hoffman described the move as a “fairly routine but proactive measure” to ensure that materials are available to interested parties — like Congress.

When I asked the Pentagon why Hood (and not Ney) responded to the subpoena, I got no response.

Asked point-blank on Oct. 13 on “Face the Nation” about the congressional subpoena, Defense Secretary Mark T. Esper said: “We will do everything we can to comply.”

Pressed by host Margaret Brennan if that was a yes, Esper replied: “That’s a yes.”

Yet two days later, the Pentagon refused to turn over any information, citing in part a legal argument that had been rejected by the U.S. Court of Appeals for the District of Columbia Circuit four days earlier.

Esper is not in an easy position. The Senate confirmed him on July 23, two days before Trump’s infamous phone call with Volodymyr Zelenskiy, during which Trump put pressure on his Ukrainian counterpart to investigate former Vice President Joseph R. Biden, Jr., a chief political rival. Pentagon spokesman Hoffman confirmed that Esper was not on the July 25 call, but questions remain about what the Pentagon knew about the hold placed on the aid, and when it knew it.

Put another way: Was the Defense Department complicit in withholding congressionally appropriated aid to a partner nation, or was Esper cut out of the loop as the White House made crucial decisions on national security? Nodding along in agreement, or caught unawares after the fact? 

The answers to these questions are likely to be found in the materials that the Pentagon now won’t disclose, a decision at odds with Esper’s promises of candor and transparency.

Indeed, the Associated Press reported that he told House investigators that he intended to cooperate with the impeachment inquiry, only to be overruled at the last minute by the White House.

Congress may be sympathetic to a good faith effort to review the relevant documents to screen for classified information, as Hood’s letter suggests. But if the Pentagon’s response is a genuine — and partisan — challenge to Congress’s ability to conduct oversight, that doesn’t bode well for Esper’s relationship with the Hill.

“It is not appropriate to ignore legitimate requests from Congress necessary to execute its constitutionally granted authority to conduct oversight,” House Armed Services Chairman Adam Smith, a Washington Democrat, said in a statement. “Regardless of the Administration, the Department of Defense has always responded to the oversight requests of Congress and we expect that to continue.”

Simply put, making promises it can’t keep should not become standard operating procedure for the Pentagon.