BAGHDAD - Shiite militiamen are everywhere. Police and Iraqi army checkpoints are nowhere in sight. U.S. soldiers are keeping their distance. Sadr City — the Baghdad nerve center for the powerful Mahdi Army — is suddenly back on edge as the militia leader, Muqtada al-Sadr, and Iraq's government lock in a dangerous confrontation over clout and control among the nation's majority Shiites. The epicenter of the showdown has been the southern oil hub of Basra, where clashes have claimed dozens of lives this week and al-Sadr's forces face a Friday deadline to surrender. But a more finely tuned measure of...