Britain's Queen Elizabeth II (2nd L) and her husband Prince Philip, Duke of Edinburgh (L) greet US President Barack Obama (R) after he and his wife Michelle arrived by helicopter outside Windsor Castle, west of 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 PHOTO / Jim Watson