Russia signaled deepening intervention Monday in the Syria war, strongly hinting that its “volunteer” ground forces would soon be fighting there, as NATO officials warned the Kremlin after a Russian warplane invaded Turkey ’s airspace. Russia called the air incursion an innocent mistake because of foul weather, a claim that the Americans rejected. The unfolding developments reflected a dangerous new superpower entanglement in the war, which has left a quarter-million people dead and half the country’s population displaced since it began more than four years ago. The addition of Russia ground forces to the assaults already underway by Russian warplanes particularly threatens to undermine Turkey’s Syria policy, which aims for the establishment of a “safe zone” along the Turkish border where some Syrian refugees could return in the future. Russia and Iran have moved aggressively in the past few weeks to strengthen their ally, President Bashar al-Assad […]