In early August, 2004, Sistani experienced serious health complications related to a previously diagnosed heart condition. He traveled to London to receive medical treatment. It was, reportedly, the first time that Sistani had left Iraq in decades, and may have been due, in part, to growing concerns for his safety from sectarian violence. Though still recovering, Sistani returned later in the month to broker a military truce at the Imam Ali Mosque in Najaf where Muqtada al-Sadr and the Mahdi Army had been cornered by American and Iraqi forces. Sadr, who rose rapidly to prominence through a series of independent military actions beginning in 2004, has since actively challenged Sistani's more progressive influence over Shia in the region.