The Krasnodar, a Russian attack submarine, left the coast of Libya in late May, headed east across the Mediterranean, then slipped undersea, quiet as a mouse. Then, it fired a volley of cruise missiles into Syria.

In the days that followed, the diesel-electric sub was pursued by the aircraft carrier USS George H.W. Bush, its five accompanying warships, MH-60R Seahawk helicopters and P-8 Poseidon anti-sub jets flying out of Italy.

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