Many Democrats have a hard time understanding why Republicans want to keep investigating the Sept. 11, 2012, terrorist attack in Benghazi, Libya.

Some see the GOP trying to score partisan points for this November's midterms. Others see a plot to undermine Hillary Clinton's 2016 prospects. Still others see Republican psychosis, the Benghazi variant of Obama Derangement Syndrome.

“ The committee concluded the Pentagon did everything it could with what it had on that ugly night. ”

In fact, the Benghazi controversy, rather than being an all-out political war, is a limited conflict in which some parts of the administration have cooperated with Congress while others haven't.

Republican sources on Capitol Hill say that in general, the Pentagon's cooperation has been a model of how to deal with such an investigation, while the State Department and White House have been models of what not to do.

If the rest of the administration had followed the military's example, the Benghazi controversy would likely be over by now.

The probe started with three questions. One, was the U.S. adequately prepared for possible trouble abroad on the anniversary of Sept. 11?

Two, did the government do everything it could to try to rescue the Americans who were under attack for seven and a half hours?

And three, did the Obama administration tell the straight story about what happened?

Responsibility for answering the first and third questions fell heavily on the State Department and the White House.

In general, their response has been incomplete, unreliable, confrontational, and deeply frustrating for investigators trying to piece together the Benghazi puzzle.

But responsibility for answering the second question, about the immediate response to the attack, fell mostly to the Pentagon. And that has been an entirely different story.

The military side of the investigation was done mostly by the House Armed Services Committee. The interim report from majority Republicans on the committee, released in February, found that the military response to Benghazi was severely hampered because every significant U.S. military asset was out of position to respond on Sept. 11.

Nevertheless, the committee concluded the Pentagon did everything it could with what it had on that ugly night.

"The regional and global force posture assumed by the military on Sept. 11, 2012, limited the response," committee Republicans wrote.

Given that, the investigators said, "members have not yet discerned any response alternatives that could have likely changed the outcome of the Benghazi attack."

Interviewing sources up and down the chain of command, Republicans went over all the military options that were activated or considered to aid the Americans under attack: two FAST platoons of Marines in Spain; a group called the Commander's In-Extremis Force, in Croatia; a special operations unit based in the U.S.; a fighter jet flyover; an armed drone; and more.

The GOP lawmakers came away satisfied that they saw everything; for example, when they asked whether AC-130 gunships could have been used, the Pentagon provided the location of every single AC-130 in the U.S. fleet at that time.

Another example was the possibility of a flyover. Even with other U.S. forces out of place, many lawmakers wondered whether American fighter jets could have buzzed the scene at Benghazi, possibly distracting and scattering the terrorists.

Air Force officials carefully walked the committee through every option that was available, and the factors -- capacity for refueling, overflight permissions, nighttime guidance, the prevalence of shoulder-fired anti-aircraft missiles in Libya, and more -- that went into the decision not to scramble the jets.

After much thought and investigation, committee Republicans concluded a flyover "would probably have been ineffective" and that the Pentagon's decision "makes sense."

In the end, Republicans had few, if any, complaints about the military's cooperation. "They were very responsive," says a GOP committee aide.

"There wasn't a witness we weren't allowed to talk to. There was never an inappropriate delay in providing the documents we wanted to see. Their response was, as far as we're concerned, timely and complete."

What a contrast to the rest of the Obama administration. First, there was the attempt to blame the attack on outrage over an anti- Muslim Internet video -- a claim that was quickly discounted by everyone who has investigated the matter.

Then the State Department conducted an internal review that seemed designed, in part, to build a firewall around then-Secretary of State Hillary Clinton.

Then the administration failed to make available a single witness who was actually on the ground in Benghazi the night of the attack.

And recently, of course, Republican complaints that the administration has withheld documents were dramatically confirmed by revelations of emails the administration failed to produce to Congress.

The administration's stance long ago exhausted whatever patience existed among Hill Republicans. Members are "really tired of getting jerked around," another aide said recently. That's a succinct explanation for the creation of the new select committee.

But it didn't have to be that way. The rest of the administration could have cooperated like the Defense Department. If it had, Washington would be talking about something else now. Instead, the fight goes on.