By Robin Emmott, Tom Balmforth and Vladimir Soldatkin BRUSSELS/MOSCOW (Reuters) -Britain said on Thursday the West could face the “most dangerous moment” in its standoff with Moscow in the next few days, as Russia held military exercises in Belarus and the Black Sea following its troop buildup near Ukraine. Tensions remained high, with Ukraine also staging war games, but leaders on all sides signalled they hoped diplomacy could still prevail in what British Prime Minister Boris Johnson called Europe’s biggest security crisis for decades. In a new round of diplomacy, Britain’s foreign minister …