Chinese leader Xi Jinping gave U.S. President Donald Trump an ultimatum of sorts when they met in Beijing Thursday to begin a two-day, high-profile summit. In talks at the Great Hall of the People in Beijing’s Tiananmen Square, Mr. Xi held out the prospect of a new, “constructive” U.S.-China relationship with stronger economic and trade ties – as well as more talks between diplomats and military chiefs. But Mr. Xi also issued a strong warning to Mr. Trump. Sound U.S.-China relations, he said, depend above all upon how the two sides handle one vital issue: Taiwan. Why We Wrote This Talks between Chinese leader Xi Jinping and U.S. President Donald Trump on Thursday struck a positive tone, as Mr. Trump avoided sensitive issues and focused instead on business. “The U.S. must handle the Taiwan issue with the utmost caution,” Mr. Xi told Mr. Trump, according to the official Xinhua News Agency. If handled well, the relations will remain stable.…