While Turkey’s deepening authoritarianism alone should be enough to disqualify it from NATO, its recent behaviour is of at least equal concern. While perhaps not overtly complicit with the so-called Islamic State (IS) as some have charged, it passively enabled it by failing to seal off the flow of jihadists to, and oil from, IS-controlled territory. More recently, it has intervened militarily in Syria for the sole purpose of preventing the most effective anti-ISIS fighting force – the Kurdish YPG, armed and trained by the United States – from consolidating territory along its southern border. Far from contributing to a peaceful solution to the tragic situation in Syria, Ankara is inflaming it.

In 2017, Turkey also broke ranks by purchasing an advanced Russian surface-to-air missile system, the S‑400, over strong objections from fellow NATO countries. The deal not only benefits NATO’s chief strategic competitor but threatens to undermine the interoperability on which NATO’s military effectiveness depends.

It is possible, of course, that a future Turkish leader will right the ship and bring Turkey back to the fold. Mr. Erdogan himself is entirely to blame.