VILNIUS, Lithuania — Dozens of United States Special Operations forces are now in the Baltics to bolster the training and resolve of troops who are confronting a looming threat from Russia, and to enhance the Americans’ ability to detect Moscow’s shadowy efforts to destabilize the former Soviet republics. “They’re scared to death of Russia,” Gen. Raymond T. Thomas, the head of the Pentagon’s Special Operations Command, who visited here recently, said of the tiny militaries of Latvia,...