How do you deal with “allies” who are stabbing you in the back on security issues even as they claim they are helping?

Think Pakistan — which received bipartisan U.S. backing for decades even as it sheltered the Taliban and Osama bin Laden. Or Saudi Arabia, whose citizens and charities still fund jihadis.

Or a Turkish government that is endangering U.S. and European security due to the political ambitions of President Recep Tayyip Erdogan.

The blinders have finally come off with regard to the first two “frenemies.” When a U.S. drone strike killed the top Taliban leader on Pakistani soil last weekend, President Barack Obama was sending Pakistan’s leaders a message: Washington has lost patience with their support for the Taliban (not to mention Osama bin Laden).

Meantime, Congress is now squeezing Riyadh to come clean on any Saudi officials who helped 9/11 hijackers. Now it’s time for a reality check on Turkey.

When it comes to fighting the Islamic State, Erdogan has long been playing a double game.

Once hailed as the model of a moderate Muslim democrat, the Turkish leader has become increasingly Islamist and antidemocratic. After initiating promising peace talks with Turkey’s Kurdish PKK rebels, he has restarted a bloody civil war with the Kurds for domestic political reasons. That battle not only undermines Turkish democracy but harms the fight against the Islamic State as well.

Having muzzled Turkish media, Erdogan is now seeking to crush all his domestic political opponents. Toward that end, he engineered a parliamentary vote last week that lifts immunity from dozens of parliamentary deputies who are facing criminal charges — mostly for speaking out against government repression.

This move is mainly aimed at destroying Turkey’s third-largest political party, the Kurdish People’s Democratic Party, or HDP. Avowedly secular and pluralist, and headed by the charismatic Selahattin Demirtas, the HDP attracted many non-Kurds. Its growing strength had prevented Erdogan from obtaining sufficient parliamentary votes to change the Turkish constitution and transform a ceremonial presidency into an all-powerful post.

“This is not just a matter of the Kurds, it is a matter of destroying democracy totally,” says Mehmet Yuksel, the HDP spokesman in Washington. “Erdogan is trying to get absolute power.”

The Turkish leader has played a clever international game, positioning himself as an essential NATO ally — which distracts attention from his domestic actions. Yet a closer look reveals that this alliance is undercut by Erdogan’s double dealing on the refugee issue — and on the fight against the Islamic State as well.

First, the refugees. The European Union, desperate to block new waves of Syrian refugees, agreed to pay Erdogan billions of dollars and grant Turks visa waivers. In return, Turkey has agreed to take back large numbers of refugees from Greece and block new ones from crossing.

However, Erdogan isn’t likely to meet the EU’s human-rights conditions for the visa waiver, which will probably doom the accord. Moreover, if the visa waiver does go through, Erdogan’s crackdown on Turkish Kurds is likely to drive tens of thousands of them to emigrate, legally, into Europe.

Huge numbers of Kurdish civilians are being driven out of Turkey’s southeast by draconian military sweeps that reportedly include massacres — notably in the town of Cizre.

In other words Erdogan’s game will cost Europe dearly.

When it comes to U.S. security interests, Erdogan is also dissembling. While claiming to play a major anti-Islamic State role, Turkey long let foreign fighters cross its border into Syria to join the jihadis. It also let Islamic State cadres cross back into Turkey for medical treatment or a break from the fighting. Due to this open-border policy, the jihadi group was also able to establish a network of supporters inside Turkey.

At the same time, Ankara is interfering with U.S. strategy to roll back the Islamic State inside Syria. Since restarting the war with its own Kurds, Erdogan hotly opposes U.S. air support and training for Syrian Kurdish fighters who are the most effective force in fighting the Islamic State. The Turks have even shelled Syrian Kurdish troops and civilians.

Fortunately, the Obama administration hasn’t let Turkish protests dissuade it from helping the Syrian Kurds. However, Erdogan’s obsession with crushing the Kurds — at home and in Syria — undercuts any coherent coalition strategy for defeating the Islamic State. It also thwarts any coalition effort to devise a workable strategy to end the Syrian civil war.

So it’s long past time for the White House and EU leaders to warn Erdogan that he is endangering Western security. His policies have created so many external enemies for Turkey that he can’t afford a complete break with his NATO allies. That gives the allies leverage.

Having signaled Pakistan and the Saudis that we’re on to their tricks, it’s time to do the same with Erdogan, and soon.

— Trudy Rubin is a columnist and editorial-board member for the Philadelphia Inquirer.