Ankara’s purchase of Russian S‑400 missiles, despite the vehement objections of the United States and other NATO members, has led to new calls to expel Turkey from the alliance. Such calls have surfaced before, mostly in response to the country’s mounting authoritarianism under President Recep Tayyip Erdogan, but this time the anger is deeper and more widespread. Moreover, the complaints stress not only Ankara’s domestic misdeeds but also worries that NATO has a dangerously unreliable partner on security policy. Washington’s decision to oust Turkey from further participation in the F‑35 fighter program certainly reflects U.S. uneasiness.

The issue of Turkey’s autocratic behavior raises fundamental questions about NATO’s standards and priorities in the 21st century. During the Cold War, the alliance’s goals were straightforward. Deterring possible Soviet aggression was the primary mission. Securing greater unity among Western Europe’s democracies, preventing the re-nationalization of defenses and consolidating the United States’ security commitment to Europe followed close behind.

A strong internal commitment to democracy was desirable but not essential for membership. Indeed, one of the founding members, Portugal, was an outright autocracy under President António Salazar. When Turkey and Greece became members in 1952, standards of internal governance became even less rigorous. Turkey’s military was a key power behind the scenes until the beginning of the 21st century, despite the prevalence of ostensible civilian rule. Greece became a full-blown dictatorship for seven years when a cabal of colonels seized power in 1967. Yet there were no serious moves to ostracize, much less expel, either country. Maintaining the alliance’s security solidarity was deemed too important to tolerate such a disruption.

In the post-Cold War era, though, Western leaders routinely portray NATO not merely as a military alliance but also as a league of democracies. Turkey’s mounting domestic repression has become an acute embarrassment. Erdogan has consolidated an alarming degree of power in the office of president, undermined the country’s once-independent judiciary, arranged for political cronies to purchase the most influential media outlets, and jailed hundreds of independent journalists and political opponents. He used an abortive military coup in July 2016 as a pretext to purge the military, the courts and the educational system of individuals he considered adversaries. Although elections continue to be held — including a crucial one last month in which voters chose an Erdogan opponent as the mayor of Istanbul — it is increasingly difficult to consider Turkey a genuine democracy.

Even more worrisome, other NATO members are showing similar signs of authoritarianism, although not as far advanced. Viktor Orban, Hungary’s prime minister, has adopted a variety of measures to harass political opponents and weaken the independence of the country’s judiciary and media. He also has expressed personal admiration for autocratic political systems, such as those in Singapore, China and Russia. Poland’s right-wing government is taking steps to bring that country’s judiciary under partisan political control and stifle public criticism of regime policies.

Such developments mean that Western leaders must determine whether NATO is purely a security organization or whether members also must abide by fundamental standards of human rights and democratic governance. Turkey indisputably is failing to live up to such standards, and the trends in both Hungary and Poland are alarming. NATO’s leaders cannot evade the question of the alliance’s identity much longer.

Given Ankara’s external conduct, the other NATO members also cannot avoid the question of whether Turkey is a reasonably reliable security partner. The S‑400 purchase was an ostentatious snub of alliance policy. Among other problems, it is unlikely that those weapons can be integrated into NATO’s overall air defenses. Moreover, the missile deal is simply the latest example of Erdogan’s growing rapprochement with Vladimir Putin’s government. It is increasingly doubtful, for example, whether Ankara will continue to support the array of economic sanctions that the Western powers imposed on Moscow to retaliate for Putin’s annexation of Crimea.

Washington is reluctant to support Turkey’s expulsion from NATO or otherwise sever security ties with Ankara. U.S. leaders have long considered that country a linchpin on NATO’s southeastern flank and a vital player in the volatile Middle East. Continued U.S. access to the Incirlik air base also is seen as a crucial element of Washington’s force-projection capabilities throughout that region — an especially important consideration as U.S. relations with Iran continue to deteriorate.

But while Incirlik is a valuable military asset, it is not irreplaceable. Washington also deploys powerful, carrier-based aircraft. Moreover, there is no certainty that Ankara would permit use of the base for any mission Washington wished to pursue. That uncertainty is likely to grow if U.S. and Turkish interests and policy preferences continue to diverge.

In any case, access to Incirlik is not a sufficient reason for the United States to support retaining an authoritarian member in what purports to be a democratic alliance. It certainly is not an appropriate reason for retaining an unreliable, duplicitous security partner. Turkey no longer is a credible or desirable ally on the basis of either political values or security considerations. The United States and NATO need to part ways with Ankara.