As chaos spreads in the aftermath of Russia’s invasion of Ukraine, President Biden is dispatching Vice President Harris to Eastern Europe on an urgent mission this week to reinforce Western unity, reassure allies of U.S. protection and promise aid as more than 1 million Ukrainian refugees flee their homes. Harris will meet with leaders in Poland on Thursday and their counterparts in Romania a day later, in what amounts to an extension of her recent trip to a global security conference in Germany. There, in the earliest days of the Russian invasion, Harris met with an array of European and...