President Obama warned Vladimir Putin of "additional costs" for Russia after a weekend referendum to separate Crimea from the Ukraine went forward. In a phone call between the leaders on Sunday, Obama told the Russian president that the U.S. and its allies were "prepared to impose additional costs on Russia for its actions." And the U.S. signaled it could act as soon as Monday to impose sanctions against Russia for its incursion into Crimea, and for conducting the referendum while occupying troops were in place. The referendum to break Crimea away from Ukraine and make it a part of Russia...