WASHINGTON — THE fall of the Iraqi city of Ramadi on Sunday, and of the Syrian city of Palmyra on Wednesday, is a big gain for the Islamic State, but not an utter disaster, as many observers fear. Rather than inducing panic in Western capitals, it should lead to a realistic assessment of the Islamic State’s strengths and weaknesses. One setback in a long war must not trigger hasty strategic shifts that lead to foreign countries’ becoming mired in Iraq once more. Palmyra has economic and cultural significance, as it sits among gas fields and is home to renowned ruins....