Defeating the Islamic State was candidate Trump’s top national-security priority, one of the few policy issues on which he was consistent. While his claim to have a secret plan — and that keeping it secret was good strategy — was risible to national security experts, his policy goals were and are consistent. American effort should focus on fighting the Islamic State. Regime change to push Bashar al-Assad out of power was not only a lesser objective, but counterproductive to a stable end-state for Syria that prevents terrorism and too costly given Russia and Iran’s support for the regime. Stability is...