ABOARD THE USS HARRY S. TRUMAN—This 20-story-tall aircraft carrier with a crew of 5,000 made an unplanned diversion from the Gulf to the eastern Mediterranean last week—a quick pivot intended to send a clear message to Russia.

The massive ship serves as a launching point for a near-constant barrage of airstrikes on Islamic State targets in Iraq and Syria. Since November, it has accounted for a little more than half of the total sorties flown over those two countries by the U.S. military.

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