A knife-wielding intruder allowed to run rampant through the first floor of the White House. Bullets that struck the window of Barack Obama's private residence but went unnoticed for days. A presidential bodyguard so drunk he passed out in hallway of a hotel. These are just some of the recent incidents that have shaken confidence in the Secret Service, the elite agency assigned to protect Mr Obama, his family and the White House. On Tuesday, Julia Pierson, the Secret Service's director, will face a grilling from members of Congress and a battle to convince them that her agents are up...