British Prime Minister David Cameron (R) poses for pictures with US President Barack Obama before a meeting in 10 Downing Street in central London on April 22, 2016.      
US President Barack Obama plunged into Britain's increasingly poisonous EU debate on Friday at the start of a visit, warning strongly against Brexit and pointing out that US soldiers had died for Europe. / AFP / Jim Watson        (Photo credit should read JIM WATSON/AFP/Getty Images)