U.S. President Joe Biden on Saturday said the war in Ukraine has been a “strategic failure” for Russia and that Russian President Vladimir Putin “can and must end this war.” Biden also said Putin “cannot remain in power,” a remark that a White House official later clarified was not intended as a call for regime change. Biden was in Poland on the second day of his trip to the country—also the fourth and final day of his trip to Europe. In a major address delivered from Warsaw, the U.S. president warned Russia against encroaching on NATO territory. “Don’t even think about moving on one single inch of NATO territory,” Biden said. “We have a sacred obligation under Article 5 to defend each and every inch of NATO territory with the full force of our collective power.” According to the United Nations, of the more than 3.7 million people who have fled Ukraine …