(Reuters) – Russia has massed troops near Ukraine and the West has threatened economic sanctions if it launches an invasion. Moscow denies planning an invasion but has demanded sweeping Western security guarantees in a tense standoff. Here is a timeline of the main events in Ukraine’s political history since it won independence from Moscow in 1991. * 1991: Leonid Kravchuk, leader of the Soviet republic of Ukraine, declares independence from Moscow. In a referendum and presidential election, Ukrainians approve independence and elect Kravchuk president. * 1994: Kravchuk loses a presidential elec…