The Cold War is history, but its spirit this weekend stalked the security conference held here each winter for the past 51 years. Chancellor Angela Merkel of Germany and Senator John McCain drew very different lessons from the West’s 20th-century showdown with the Soviet Union, as they clashed over whether to arm Kiev’s troops in response to Russian aggression in eastern Ukraine . That disagreement represents one of the sharpest rifts yet over how the allies should respond to the current challenge from Moscow. Europe is very different from the days when the militaries of NATO and the Soviet bloc stared each other down. But the crisis in Ukraine has reopened familiar differences as to whether the threat of force — in this case, Washington’s readiness to bolster Ukraine’s military against pro-Russian rebels backed by the Kremlin — is more effective than months of so-far failed negotiations. […]