BAGHDAD — Iraqi Shiite cleric Muqtada al-Sadr, whose coalition won the largest number of seats in last month’s parliamentary elections, has announced an alliance with an Iran-backed coalition ahead of marathon negotiations to form a new government. The move, announced by al-Sadr and Hadi al-Amiri of the Fatah coalition in the revered southern Shiite city of Najaf, came largely as a surprise as al-Sadr has been touting himself as a nationalist leader who opposes Iranian influence in Iraq. The new alliance controls 101 seats, still far from the 165 required for a majority. At a news conference Tuesday, both leaders underscored that their alliance is aimed at expediting the formation of a new government and they called on others to join them. “We had a very positive meeting in order to end the suffering of the country and the people,” al-Sade said.