PARIS — NATO is ready to engage in a potential Arctic mission, but hasn’t started planning one as there has been no political guidance to do so, said Gen. Alexus Grynkewich, the Supreme Allied Commander Europe, after a meeting of the organization’s top military leaders on Thursday. “We’re aware of discussions about this, of course, and therefore we’re doing some thinking about how we would organize for it, but no planning has started yet,” said Grynkewich, a United States Air Force general, at