Three years after President Obama promised he would “always have Israel’s back,” it’s clear he meant that Israel should “watch your back.”

Obama’s 2012 vow was an election-year ploy that is now revealed as nakedly false. Every American president since Harry Truman has had a dust-up or two with their Israeli counterparts, but no president consistently has tilted toward Israel’s enemies the way Obama has.

His apology tours to Muslim nations and his pro-Palestinian stance, especially his demand that Israel negotiate with Hamas terrorists, were foolish, but carried no lasting impact and could be seen as naive mistakes.

But no such forgiving explanation is possible for Obama’s six-year deferential courting of Iran’s mad mullahs and his correspondingly long animosity toward Prime Minister Benjamin Netanyahu.

Despite the many tyrants, genocidal maniacs and dimwits who lead other nations, only Israel’s leader is singled out repeatedly for Obama’s wrath.

This is a peculiar form of prejudice, one Obama embraces with too much relish.

The result is not a simple matter of temporary daylight between two great allies over a principled conflict. This is a ruthless turning on a special friend, one whose existence is threatened daily by our shared enemy, Islamic terrorism.

Predictably, the White House wants to paint the tension over the Netanyahu visit as a partisan extension of Obama’s war with Republicans. Just as predictably, some black Democrats play the race card and claim the speech to Congress is a sign of disrespect to Obama personally.

Neither passes the smell test. The only thing that matters is whether Obama’s emerging deal with Iran is a death knell for Israel and a threat to Western civilization.

If Obama really believes it’s a good deal, he should welcome a debate over its terms. Instead, he uses artifice and threats to silence disagreement.

Under any other president, it would be inconceivable that the United States of America and Iran would be united in support of a nuclear agreement that Israel sees as an existential threat. Yet Obama is joined at the hip with a founding member of the Axis of Evil.

The moment marks the ultimate demonstration of how completely he has reversed American policy. Enemies are now partners, and friends are betrayed and abandoned.

Israel is not the only loser in this historic switch. Our closest Arab allies, including Saudi Arabia, Egypt and Jordan, are also at odds with Obama over his tilt toward Iran. Despite their societies being riven with anti-Semitism, they fear Iran more than Israel. Indeed, they have lived for generations next to an Israeli nuke, but say they cannot accept an Iranian nuke.

Those and other vulnerable Muslim nations also are alarmed at Obama’s refusal to lead the fight against the spreading cancer of the Islamic State. In a development that goes largely unrecognized, each is looking to other nations — France and China mostly — for expanded trade and military supplies.

In terms of advancing America’s influence and national security, none of this makes a whit of sense outside the Obama bubble. But he insists the Islamic State is not Islamic and sees Iran as a potential force for “regional stability,” two fantasies that no other Western authority shares.

In reality, the Islamic Republic and the Islamic State have much in common, despite their sectarian divide. Iran remains the world’s largest sponsor of terrorism, with proxies such as Hezbollah killing our soldiers in Lebanon and Iraq. It, too, is a theocracy bent on religious domination, yet it gets more trust and respect from this White House than our most loyal democratic ally in the Mideast.

Israel is sometimes called the 51st state — but the president’s top national security adviser, Susan Rice, goes on television to insist Netanyahu’s visit is “destructive” to the two nations’ relationship and Obama refuses to meet him.

Even Vladimir Putin suffers no such derision. As he rains death and destruction on Ukraine and threatens NATO, John Kerry, who attacked Netanyahu in congressional testimony last week, met yesterday with the Russian foreign secretary in Switzerland. There, Putin puppet Sergei Lavrov continued to insist there are no Russian soldiers in Ukraine. Kerry offered no public criticism, only cheerful tweets about a cease-fire that Russia keeps violating.

There is some logic to the linkage of Netanyahu to Republicans, who seem to be Obama’s only true enemy. Netanyahu’s party is akin philosophically to the GOP, which is why he was invited to warn against the Iran deal, and he favored Mitt Romney in 2012.

But that logic breaks down when you consider that Iran just last week blew up a replica of an American military ship. Two weeks earlier, demonstrators at an officially approved rally in Tehran marched in front of an Obama effigy hanging from a gallows, Fox News reports.

It said the Iranians chanted, “Death to America” and “Death to Israel,” and burned and trampled an American flag.

In response, our president is prepared to allow them to have a nuclear weapon — and demonizes Israel and anyone else who refuses to agree.

Just whose side is he on?