ISTANBUL – Turkey’s Prime Minister Recep Tayyip Erdoğan has taken on a daunting challenge. After participating in the nuclear-security summit in South Korea at the end of March, he went to Tehran to urge Iran’s leaders to make a deal during the next round of nuclear talks between Iran and the United Nations Security Council’s five permanent members (Britain, China, France, Russia, and the United States) plus Germany. And Erdoğan will host those talks in Istanbul in mid-April.