Stocks on Wall Street shed early gains and closed broadly lower Monday as the United States moved to close its embassy in Ukraine amid heightened tensions over the thousands of Russian troops that have been amassing on the border. The S&P 500 fell 0.4 percent after having been down as much as 1.2 percent shortly after the United States said it is closing its embassy in Ukraine and moving all remaining staffers there to a city near the Polish border. The move comes as diplomatic efforts continued Monday in a bid to head off what U.S. officials have warned could be an imminent Russian attack on Ukraine. Bond yields rose broadly, as did energy futures and the price of gold. The Dow Jones Industrial Average fell 0.5 percent and the Nasdaq composite ended essentially flat after having been up 1 percent in the early going. The three major stock indexes …