Not long ago it would have seemed unthinkable to add Turkey to the list of countries — including North Korea, Iran and Russia — that the United States had sanctioned for unscrupulous behavior. As a NATO ally, Turkey has a mutual defense treaty with Washington, benefits from American intelligence and hosts American nuclear weapons at Incirlik air base, near its border with Syria.

As August began , however, President Trump named Turkey’s interior and justice ministers as “specially designated nationals ” barred from doing busines s with Americans and gaining access to financial assets in the United States. On Friday , Mr. Trump announced in a tweet that he had authorized a doubling of the steel and aluminum tariffs against Turkey.

The object is to force the Turkish president, Recep Tayyip Erdogan, to release Andrew Brunson, an American and evangelical Christian pastor who has been imprisoned by Turkey since 2016 on trumped-up charges of aiding an aborted coup by Erdogan opponents.

The current tension is a far cry from the camaraderie expressed at the NATO meeting last month, when Mr. Trump fist- bumped Mr. Erdogan, previously one of his favorite strongmen. But it’s also the latest example of how dangerously frayed the United States-Turkey relationship, with its accumulating resentments, has become over the past decade, giving rise to the question: Is Turkey still an American ally?