John R. Bolton -- who is seeking confirmation as the next U.S. ambassador to the United Nations -- often blocked then-Secretary of State Colin L. Powell and, on one occasion, his successor, Condoleezza Rice, from receiving information vital to U.S. strategies on Iran, according to current and former officials who have worked with Bolton. In some cases, career officials found back channels to Powell or his deputy, Richard L. Armitage, who encouraged assistant secretaries to bring information directly to him. In other cases, the information was delayed for weeks or simply did not get through. The officials, who would discuss...