Turkey’s military strike on Wednesday against Kurdish fighters in northern Syria — a group long regarded as the United States’ most reliable Syrian partner in its fight against the Islamic State — has been years in the making.

To Turkey, an important American ally and NATO member, those fighters are terrorists bent on undermining the Turkish government. But for the United States, they have been dependable allies who have died fighting alongside American soldiers.

Since 2014, with the start of military action against ISIS, Kurdish fighters have done the critical work of clearing and holding territory conquered by the militants. In recent years, they have taken care of refugees and detained terrorism suspects.