President Trump has approved a plan to arm Syrian Kurds so they can participate in the battle to retake Raqqa from the Islamic State, a strategy that has drawn deep opposition from Turkey, a NATO ally.

American military commanders have long argued that arming the Y.P.G., a Kurdish militia fighting alongside Syrian Arab forces against the Islamic State, is the fastest way to seize Raqqa, the capital of the militants’ self-proclaimed caliphate.

And Mr. Trump, who made fighting Islamist militants a priority during his campaign, again showed the high regard he has for Pentagon generals by endorsing their advice when faced with a policy dilemma. […]