MOSCOW—Russian military helicopters based in Tajikistan fired air-to-surface missiles during a training exercise on Tuesday as Moscow said its forces in the Central Asian nation were fully equipped to help secure the border with Afghanistan. Tajik President Emomali Rakhmon on Monday ordered the mobilization of 20,000 military reservists to bolster the border with Afghanistan after more than 1,000 Afghan security personnel fled across the frontier in response to Taliban terrorist advances. Russian President Vladimir Putin told Rakhmon on Monday that Moscow would help the impoverished former Soviet republic contend with the fallout from NATO’s exit from neighboring Afghanistan if necessary. Russia, which operates one of its largest military bases abroad in Tajikistan equipped with tanks, helicopters, drones, and ground attack aircraft, would help stabilize the border with Afghanistan if needed, both directly and through a regional security bloc, the Kremlin said in a statement. Russia’s Deputy Foreign Minister Andrei Rudenko …