"Regrets are the natural property of gray hairs," Charles Dickens opined. And while the six men in Dror Moreh’s haunting and daunting documentary "The Gatekeepers" have gray hair — or no hair at all — theirs are not the simple regrets of spent youth.

Former directors of the Shin Bet, the Israeli counter­terrorism agency charged with keeping their nation secure, these are men who oversaw the surveillance, arrest, interrogation and even assassination of suspected enemies. While their main targets were Palestinians, jihadists and Hamas, increasingly over the years, radical fundamentalist Jews who opposed any measures of peace, or any retreat from settlement expansion in the West Bank, required Shin Bet’s attention, too.

And these six men, interviewed separately, sit before Moreh’s camera and reflect with striking candor on what went wrong, the fateful decisions that backfired, the iron-fisted strategies that brought about more violence, more bloodshed, not less.

From Avraham Shalom, who ran Shin Bet from 1980 to 1986, to Yuval Diskin, who served as the secret service’s head from 2005 until 2011, these men, all from military backgrounds, speak about the futility of many of their programs, of the directives that came down from above.

In some ways, "The Gatekeepers" is like one long confession, a bureaucratic baring of the soul: the hard details of undercover operations, the airstrikes on suspected terrorist hideouts, the killing of militants and innocents. But for all the horror stories documented here — and, on occasion, re-enacted — the overriding message is sobering and clear: This is not the road to peace.

Nominated for a best documentary feature Oscar (it lost out to "Searching for Sugar Man"), "The Gatekeepers" is remarkable for the access granted first-time filmmaker Moreh, and doubly remarkable for the frankness with which Shalom and Diskin — and Yaakov Peri (1988-1995), Carmi Gillon (1994-1996), Ami Ayalon (1996-2000) and Avi Dichter (2000-2005) — describe their perceived victories and losses, and their deep-seated frustrations.

WHEN Prime Minister Yitzhak Rabin was assassinated by a Jewish extremist in 1995, it shook Israel — and the Middle East — to the core. President Bill Clinton had orchestrated a historic accord between Rabin and PLO Chairman Yasser Arafat, but after Rabin’s death, the peace process, and the self-government plans for the Palestinians, collapsed. So, too, did the infrastructure of Shin Bet, which was held responsible for the security lapse that led to Rabin’s death.

As a clear-eyed examination of a conflict that seems to have no end, "The Gatekeepers" is powerful, pro­vocative stuff. These six men have stood on the front lines, but they also stand on the blurry lines of right and wrong, of moral doubt, and self-doubt.

If vigilance and pre-emption, recompense and retaliation are not enough, the film asks, then what is?