What did he know and when did he know it? The immortal question about Richard Nixon and Water­gate should be posed to Barack Obama about Syria. What and when did he know about Vladimir Putin’s axis-of-evil coalition?

The significance is not limited to Syria. The question goes to the heart of the Iran nuclear deal, especially the timing of the congressional votes.

Imagine Obama trying to sell the Iran deal now. With Russia, Iran and Iraq working together to muscle the United States aside and defend Bashar al-Assad, the president couldn’t possibly argue that the nuke deal would help stabilize the Middle East. Nor could he argue that Russia could be trusted to help enforce ­restrictions on Iran.

The strong likelihood that Obama would have lost the Iran vote if Congress knew then what the world knows now suggests the possibility the president concealed the Russian plan until the Iran deal was done. That view fits with his single-minded determination to get a deal at any price, including making key concessions and downplaying Iranian threats to Israel and the United States.

After all that, what’s another lie?

That view is also supported by the chronology, which reveals strong evidence the president hid the truth.

For much of September, reports of Russia moving soldiers and military equipment into Syria invariably said the Pentagon was “puzzled” or the White House was “unclear” about Putin’s intent. Obama declared on Sept. 11 that whatever the dictator’s plan, it was “doomed to fail.”

The claims of fuzziness about Syria allowed Obama to keep the focus on his push to sell the Iran pact to Congress. He touted Russia’s support, vowed to impose “snapback” sanctions if Iran cheated and said he would work to stop the mullahs’ ­regional aggressions.

His arguments and arm-twisting kept 42 Senate Democrats in line, enough to save the deal. Yet soon ­after opponents lost their final vote, on Sept. 17, Russia revealed that it would lead a coalition of Iran and Iraq to intervene militarily to save the Assad regime.

The shock-and-awe attacks launched last week are rattling the world as Russian airstrikes pound Syrian rebels, including some we support, with Iraq and Iran providing boots on the ground. But it’s not possible that nobody in Washington saw this coming.

After all, the Russian plan took shape well before late September. The Iran deal was officially finalized on July 14, and Iranian Quds Force commander Qassem Soleimani met with Putin in Russia on July 24. Fox News, which first reported the meeting, even had the flight numbers of Soleimani’s Iran Air flights between Moscow and Tehran.

Soleimani, banned from international travel because of links to terrorism, earlier had been spotted in Iraq, helping to defend Assad against Islamic State. Yet five days after Soleimani was in Moscow, Secretary of State John Kerry told the Senate the travel restrictions against Soleimani would never be lifted. Apparently, they would never be enforced, either.

Although Russia and Iran had separately supported Assad, Kerry never mentioned that they could be working together militarily. Yet the Institute for the Study of War, a respected think tank, reported that “available satellite imagery and open sources” showed that “the new buildup of Russian military forces in Syria began in July 2015 and accelerated considerably in late August and September.” That means the buildup began near the Soleimani visit to Putin.

The institute offered key details, including that in late August, a Russian ship unloaded armored personnel carriers. It cited another report from Syrian rebels that Russian-speaking soldiers were engaged in combat against Assad’s opponents.

Yet in early September, less than two weeks before the final Iran vote, Kerry still wondered whether the buildup reports were “accurate.” That ridiculous feint would soon morph to an acknowledgment of a buildup, but with convenient claims that nobody understood Putin’s intent.

Now, of course, everybody understands Putin’s intent and is alarmed because the war is widening and Russia has replaced the United States as the region’s top power broker, a blow to our national security and allies.

But there still is little understanding of the connection between this tectonic shift and the Iranian nuclear deal. In fact, the deal was the final piece that put the Syria plan into action.

By eliminating most sanctions and freeing Iranian assets, the nuke deal provides money and protection for the world’s largest sponsor of terrorism to attack our allies. And Iran’s liberation gave Putin the Muslim ground troops he needs.

So the question needs to be asked of Barack Hussein Nixon: What did you know, and when did you know it?

Another cop-out by Blas

Top aides are locked in a bitter battle over one of the most important issues in New York, so what does Mayor Putz do? He punts and heads out of town.

On the same day Police Commissioner Bill Bratton announced reforms to use-of-force guidelines, the NYPD’s inspector general released a study of incidents and said Bratton’s reforms fall short.

The IG, Philip Eure, accused the department of being “in the dark ages” compared to other cities.

A furious Bratton demanded an apology, but got none. At that point, a mayor who is a leader and who understands the difficulty of police work would have stepped in to defend his commissioner and his troops.

No surprise, de Blasio is not that mayor. While he repeatedly praised Bratton, the Putz ducked and weaved when asked Friday about Eure’s outrageous “dark ages” remark.

“It’s not surprising that the inspector general has an opinion,” de Blasio told an interviewer.

It’s not surprising that de Blasio is a wimp, either. He’s a fair-weather friend to cops, in their corner until something goes wrong or a charge is leveled. Then he checks to see which way the wind is blowing, or which way Al Sharpton is leaning.

The pattern is fixed, but this case is no mere difference of opinion. The city should speak with one voice on such a crucial topic, and if de Blasio can’t stand with the cops, he ought to stay out of town.

Dems’ unholy silence

It’s unanimous. Obama, Gov. Cuomo and Mayor de Blasio all denounced the “gun violence” in the Oregon slaughter — and all failed to mention that Christians were singled out for murder.

Would they have been silent on religion if the dead were Muslims?

The question answers itself. Whatever their personal faith, leading Democrats believe in only one politically correct religion.

Hillary at 400 lies per minute

It’s impossible to count Hillary Clinton’s lies about her e-mail scandal, but here’s a handy shortcut. Despite her claim that there was “no classified material” on her home-brew server, officials found 400 e-mails they now consider classified, even if they weren’t marked at the time.

Because Clinton was required to safeguard sensitive information, that makes 400 lies in one sentence.

Even for her, that’s gotta be a record. Guinness!