BAGHDAD, Iraq, June 28 — In a surprise, secret ceremony that was hastily convened to decrease the chances of more violence, United States officials today handed over sovereignty to Iraqi leaders, formally ending the American occupation two days earlier than scheduled. In a tightly guarded room behind high walls, L. Paul Bremer III, the top United States administrator, presented a formal letter recognizing Iraq's sovereignty to Iraqi Prime Minister Iyad Allawi. Just 30 or so people were present for what Dr. Allawi described as the "historic" handover. A few hours later, Mr. Bremer flew off on a military plane, leaving...