LONDON — The United States recently committed itself to arming the Kurdish People’s Protection Units, known as the Y.P.G., to help evict the Islamic State from its Syrian stronghold, Raqqa. This decision is likely to prove deeply troublesome, risking the regional stability necessary for the lasting defeat of the Islamic State.

The Y.P.G. denies that it is, in effect, a wing of the Kurdistan Workers’ Party, or P.K.K., but the evidence is clear. The P.K.K., a Marxist-leaning Kurdish nationalist organization, was founded in Turkey in 1978, and took up arms against the Turkish state in 1984. The group’s leader, Abdullah Ocalan, was expelled from Syria in 1998, when his old patron, the regime of Hafez al-Assad (Bashar’s father), came under military threat from Turkey. Mr. Ocalan was soon arrested by the Turks, and the tide of war turned against the P.K.K.

In 2003, the P.K.K. began creating branches in Syria, Iraq and Iran that did not have the burden of the P.K.K. name, to better integrate with local Kurdish populations and to avoid legal problems related to its designation as a terrorist organization by the United States, most other Western states, and international institutions like the European Union and NATO. In Syria, the P.K.K. established the Democratic Union Party, or P.Y.D.; the Y.P.G. is this party’s armed militia.

As the uprising widened in Syria during the summer of 2012, government forces retreated from areas in the north of the country, leaving the P.Y.D. in control. The Assad regime’s intention was to keep the Kurds out of the rebellion and sow dissension among antigovernment groups. Notwithstanding occasional skirmishes with Kurdish fighters, Damascus continues to underwrite the Y.P.G.-held areas, even though it opposes any long-term federalist solution for the country.