The United States has yet to begin serious negotiations with Iraqis on an agreement to guarantee that American troops in Iraq will remain immune from arrest and prosecution by local authorities once a new Baghdad government takes over in June, according to U.S. and Iraqi officials. The status-of-forces agreement, often one of the most sensitive pacts reached between the U.S. and countries that host American troops, will also set the rules for where U.S. troops will be based and the conditions under which they will operate. "All these things have to be negotiated and agreed upon -- the areas they...