After a year of trying to sideline Yasir Arafat, Secretary of State Colin L. Powell called on the Palestinian leader today to enlist the security forces under his control to help crush Hamas and other groups held responsible for the Jerusalem bus bombing on Tuesday. The unusual appeal to Mr. Arafat reflected what administration officials said was a growing realization that he remained a force to be reckoned with among Palestinians and that more pressure needed to be directed against him after the latest attack by Palestinian militants. Before today, administration officials have been reluctant even to mention Mr. Arafat’s name. Instead, they have invested their hopes in the ability of Mahmoud Abbas, the Palestinian prime minister, to work with Israel on carrying out the administration-backed peace plan toward a Palestinian state. Full Story