The Turkish president, Recep Tayyip Erdogan, bolstered by the electoral triumph of his conservative Islamist Justice and Development Party, or A.K.P., has shown a troubling penchant for benign neglect toward the jihadi Islamists — enough for them to establish a Turkish network.

What does Erdogan — in theory a key American ally leading a NATO state — see in the knife-wielding jihadis of the Islamic State? They are useful in confronting Turkey’s nemesis, the Kurds, who have taken over wide sections of northern Syria and established self-government in an area they call Rojava. That in turn has raised the specter of a border-straddling Kurdistan, the nightmare of the Turkish republic.

Hence the unpersuasive Turkish balancing act that sees Erdogan offering the United States use of Turkish air bases to fight the Islamic State even as Turkey twice strikes the positions in Syria of Kurdish militias who, as my colleague Tim Arango put it, are the “most important allies within Syria of the American-led coalition fighting the Islamic State.” Hence, also, the bungling and inaction that produced, on Oct. 10 in Ankara, the worst terrorist attack in Turkish history.