WASHINGTON — Barack Obama's plan to build up U.S. forces in Afghanistan while keeping perhaps 50,000 troops in Iraq has triggered a deep rift among antiwar activists, a reminder of the difficult tasking facing the presumptive Democratic nominee as he tries to broaden his appeal. The Illinois senator wrapped up three days of tours and talks in the war-ravaged nations Tuesday, stressing in a news conference that the "situation in Afghanistan is perilous and urgent" and that "we should not wait any longer" to provide additional troops. In Iraq , Obama won a tacit Iraqi endorsement of a plan to...