The night of Sept. 1, 2009, Echo Platoon of Navy SEAL Team 10 headed out into the Fallujah night. Their goal: concluding a five-year search for the al Qaeda killer who had been responsible for the shocking 2004 murders of four American military contractors — one of them an ex-SEAL — whose bodies were then burned, dragged through the streets and hanged from a bridge.

This night the SEALs departed with these words from their commanding officer: “Gents, stay sharp, and expect a firefight.”

In the event, no shots were fired, but the SEALs faced another kind of ambush: a humiliating, baffling, infuriating struggle with the military-justice system that would end with an unsatisfying victory.

Because the man those SEALs captured — Ahmad Hashim Abd Al-Isawi, aka “the Butcher of Fallujah,” a man who lived for mayhem — somehow sustained a bloody lip on the night of his capture.

The contrast between the two instances of violence seems, like many of the details of the case, absurd.

On the one hand, four Blackwater contractors were murdered and beheaded as they pulled security in a convoy that was attempting to deliver food.

On the other hand, Al-Isawi had a small cut on his lip that a doctor would later testify appeared to have been self-inflicted.

Yet for this latter instance of bloodshed three SEALs were charged with serious crimes. All of them — Matthew McCabe, Jonathan Keefe and one known by the pseudonym “Sam Gonzales” — were cleared, but in “Honor and Betrayal: The Untold Story of the Navy SEALs Who Captured the ‘Butcher of Fallujah’ — and the Shameful Ordeal They Later Endured” (Da Capo Press), his follow-up to his previous Navy SEALs bestseller, “Lone Survivor,” author Patrick Robinson asks why they were court-martialed in the first place.

Fair question, as is this one: If McCabe (the man who actually apprehended Al-Isawi) had wanted to abuse the prisoner, why didn’t he do so during the raid? Indeed, McCabe could simply have shot Al-Isawi (who would later hang for his crimes) and called it a night. “Shot him dead, right there in his apartment, since he was armed, with a record of crime and violence which would’ve made Genghis Khan look like Mr. Rogers.”

After Al-Isawi’s capture, he was taken into detention at the nearby Camp Schwedler, where the military policeman on duty, Master at Arms Brian Westinson, twice left the prisoner alone for a few minutes.

When presented to Iraqi police hours later, Al-Isawi had blood on his dishdasha and lip and told a preposterous tale of having been beaten and stomped by several men in boots. (The off-duty SEALs were wearing flip-flops).

But the lip injury was the only harm found on his body, and as the al Qaeda training guide known as the “Manchester Manual” (after the English city where a copy of it was discovered) advises, detainees should “always complain of mistreatment or torture while in prison.”

Westinson, though, partially supported Al-Isawi’s story: He said he saw McCabe “half-punch” the prisoner, once, though all of the other SEALs present denied this. Gonzales had already berated Westinson for leaving his post, and the SEALs came to suspect that Westinson had fabricated the story to distract from his own behavior on that night.

For weeks, their request for legal counsel was denied and the SEALs were ordered to sign confessions to lesser charges — meaning they’d be busted in pay and rank. But they refused to admit to something they didn’t do — and insisted on going to court-martial, even if a guilty verdict meant a sentence of up to a year in prison plus loss of pay and rank and dishonorable discharge.

The evidence against the three SEALs was so thin that you have to wonder if there was a hidden motivation. That hidden motivation was, of course, the prison located 18 miles from where the camp where Al-Isawi was detained: Abu Ghraib, and all of the opportunism and hysteria that those words bring to mind. Commanders were “neurotic” about Abu Ghraib, writes Robinson. All realized that any hint of unpunished abuse anywhere in their chains of command could be lethal to their careers.

In a climactic scene, Robinson recounts how the SEALs’ lawyer shredded the prosecution’s case in court. “MA3 Westinson,” said counselor Lt. Cdr. Drew Carmichael, “has told six different versions of his own story.”

(Version two, told to a superior officer a couple of hours after the bloody lip was discovered: “This is all my fault. I left my post and he got hurt. This is all going to come on me.”)

After their public humiliation, McCabe and Keefe, who are also credited as co-authors of the book, understandably no longer possessed the same fire to serve, and so the Navy and the SEALs lost two fine men who resigned with heavy hearts. (“Gonzales” remained in the Navy, which is why Robinson doesn’t reveal his name).

In an open letter to his admiral, Keefe wrote, “When I think of those complacent prosecutors and investigators, deaf to our protests, I’ll always feel that rising anger I used to reserve for the enemy.”

Chalk up another tragic loss to friendly fire: We have met the enemy, and he is us.