WASHINGTON — It was an introspective US Secretary of State John Kerry who came to the closing session of the final Saban Forum of the Obama administration on Sunday.

For four years as secretary of state, the Massachusetts politician had made the short trek to the Willard Intercontinental Hotel in early December, updating an audience that leaned Democratic – and pro-Israel – about the hottest issues on the burner, whether they were peace talks or Iran nuclear negotiations.

Introspective Kerry knew that this was his last time to explain before Washington’s think-tank elite why on his watch, the United States had once again failed to untangle the Gordian knot of the Israeli-Palestinian conflict.

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He wore reading glasses, sat rather than stood, and cited facts, figures and maps from a thick binder. And he delivered a thesis — not explicitly, but repeatedly: The United States had failed in its mission because Prime Minister Benjamin Netanyahu was unwilling to demonstrate the kind of leadership shown by his predecessors.

Kerry was at pains to stress his engagement with the prime minister. He noted that he had spoken to Netanyahu over 375 times in the past four years for a total of 130 hours. He had traveled to Israel over 40 times, and met with the premier in a number of other countries as well, he noted. But to no avail.

Kerry stressed that “Bibi and I are friends, we really are,” recalling meeting for coffee at Cambridge, Massachusetts’ Charles Hotel. But the punchline of the story was not their friendship.

“I remember him saying to me that if you are ever in a position we can get something done,” Kerry said, letting the conclusion — that he had been in such a position and yet nothing was accomplished — settle on the audience’s suited shoulders.

The failure to reach a solution — or even to slow the growth of the settlements Kerry repeatedly excoriated as “an obstacle to peace” — was, Kerry said, “a function of leadership.”

Kerry listed steps toward peace taken by Netanyahu’s predecessors, Menachem Begin, Yitzhak Rabin, Ehud Barak and Ehud Olmert. Other than Yitzhak Shamir, under whose administration the Madrid Conference established the precedent for bilateral talks and former president Shimon Peres, who he cited later, there was only one name missing from the list — Netanyahu’s.

Kerry went to lengths to describe the Obama administration’s willingness to go to bat for Israel, whether defending it at the United Nations or providing it with unprecedented levels of defense aid.

He described Palestinian Authority President Mahmoud Abbas as a leader “committed to non-violence,” and detailed US General John Allen’s near-forgotten attempts to negotiate Israeli security presence in the Jordan Valley.

But time and time again in his talk, Kerry alluded to a lack of leadership, an inability or unwillingness to surmount the risks of coalition politics for a greater end.

Not once did he take the last step and offer the sentence that he said in myriad ways: Netanyahu was not the bold leader necessary to achieve our goals in the region. The point, however, was clear.

In another six weeks, Kerry will be out of office. One of his first steps, like a long line of America’s top diplomats before him, will be to write a book in order to set the tone for his legacy.

On Sunday, he offered a preview of what the chapter on the Israeli-Palestinian conflict will sound like — and practically suggested a title for it as well: It’s Bibi’s Fault.