BAGHDAD — A string of heavy losses from powerful roadside bombs has raised new questions about the vulnerability of the Stryker, the Army's troop-carrying vehicle hailed by supporters as the key to a leaner, more mobile force. Since the Strykers went into action in violent Diyala province north of Baghdad two months ago, losses of the vehicles have been rising steadily, U.S. officials said. A single infantry company in Diyala lost five Strykers this month in less than a week, according to soldiers familiar with the losses, who spoke on condition of anonymity because they are not authorized to release...