Perhaps foreseeing a Russian win or stalemate in Ukraine, some foreign policy experts urge that it cede territory. French President Emmanuel Macron calls for not “humiliating” Russia. These entreaties could miss the mark. The battlefield is fluid and could turn for or against Ukraine. The West should prudently prepare for Ukrainian success as well as a less favorable outcome. A few weeks ago, euphoria reigned when Ukrainian forces pushed back Russian attempts to seize Kyiv, Kharkiv and Odesa — Ukraine’s three largest cities. Now, pessimism is ascendant. President Volodymyr Zelensky speaks of “very fierce” fighting in eastern Ukraine and high...