Russia warned Monday that it would take action to protect its national interests if Lithuania did not reverse a decision to bar the transit by rail of certain items destined for Kaliningrad, the small Russian exclave that is separated from the rest of the country by the Baltic states. A new potential flashpoint between Moscow and NATO emerged when Lithuania, citing incoming European Union sanctions on Russian steel and other ferrous metals, said those items could no longer be transported across its territory to Kaliningrad. Russia’s foreign ministry said it summoned a senior Lithuanian diplomat and informed her Russia viewed...