WASHINGTON—The U.S. has decided to provide pickup trucks equipped with machine guns and radios for calling in U.S. airstrikes to some moderate Syrian rebels, defense officials said. But the scope of any bombing hasn’t been worked out—a reflection of the complexities of the battlefield in Syria.

Military officials point to U.S. airstrikes, called in by Kurdish fighters, that helped drive Islamic State fighters from the city of Kobani as the model for the new campaign.

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