A letter from President Donald Trump to Iran's supreme leader has arrived in Tehran as tensions between the two countries continue to escalate. The letter, the contents of which have not been made public, was an attempt to jump-start talks over Iran's nuclear program. Trump has recently imposed new sanctions on Iran and suggested the possibility of military action, while still expressing hope for a new deal. Iran's Supreme Leader Ayatollah Ali Khamenei has mocked Trump, but his country's officials have given conflicting signals about the potential for negotiations.