Four Russian-occupied areas of Ukraine are expected on Friday to begin voting in referendums on joining Russia, in a move that raises the stakes of Moscow’s invasion seven months after fighting began. The referendums, which are illegal under international law, could pave the way for Russian annexation of the areas, allowing Moscow to frame the ongoing Ukrainian counteroffensive as an attack on Russia itself. Such a move could provide Moscow with a pretext to escalate its faltering war, which has seen Kyiv regain thousands of square miles of territory this month. In an address Wednesday, Russian President Vladimir Putin raised...