Sweden officially joined the North Atlantic Treaty Organization Thursday, a historic shift that highlights how Russian President Vladimir Putin’s war in Ukraine is transforming European security in ways he may not have foreseen. At a meeting in Washington, Swedish Prime Minister Ulf Kristersson deposited the final paperwork with Secretary of State Antony Blinken, the last step needed for the former militarily nonaligned nation to become NATO’s 32nd member. Sweden’s neighbor, Finland, joined last year. To justify his aggression in Ukraine, Putin cited the possibility of NATO expansion. Now, in one of the conflict’s many twists, his war has brought a...