Long adept at stoking unrest in the West, President Vladimir V. Putin of Russia sent troops to the Central Asian nation of Kazakhstan on Thursday to try to extinguish the latest in a series of dangerous fires to engulf the lands of the former Soviet Union, territory that Moscow views as its own sphere of influence but has struggled to keep calm. But if the turmoil in Kazakhstan has once again exposed the vulnerability of the strongman leaders the Kremlin has trusted to keep order, it has also presented Russia with yet another opportunity to reassert its influence in its former Soviet domain, one of Mr. Putin’s most cherished long-term goals. The arrival in Kazakhstan of 2,500 troops from a Russian-led military alliance amid continuing spasms of violent protest was the fourth time in just two years that Moscow has flexed its muscle in neighboring states […]