Bush Says U.S. Is to Assume Stronger Role in Ending Violence By DAVID E. SANGER [W] ASHINGTON, April 4 ? Shifting his Middle East policy after widespread criticism, President Bush announced today that he was sending Secretary of State Colin L. Powell to the region. The president also called on Israel to stop its military advances on Palestinian-controlled areas and to withdraw from the cities it has occupied in the last week. "As Israel steps back, responsible Palestinian leaders and Israel's Arab neighbors must step forward and show the world that they are truly on the side of peace," Mr....