President-elect Donald Trump said Thursday on Twitter that the United States should greatly “expand its nuclear capability,” appearing to embrace an end to decades of bipartisan presidential efforts to reduce the role of nuclear weapons in U.S. defenses and strategy. Trump’s midafternoon post may have been a response to President Vladimir Putin of Russia, who in a speech earlier Thursday called for continued improvement of his country’s nuclear abilities so it can “reliably penetrate any existing and prospective missile defense systems.”