Talks between the Syrian government and representatives of rebel factions got off to a rocky start Monday after their first face-to-face meeting in Kazakhstan that marked a major shift in the war’s dynamics and confirmed Russia’s role as regional heavyweight. The gathering in Astana, the Kazakh capital, is the latest in a long line of diplomatic initiatives aimed at ending the nearly 6-year-old civil war, which has killed hundreds of thousands of people and displaced half of Syria’s population.