Armed men overran a National Guard observation post along the U.S.-Mexico border in Arizona this week, forcing the soldiers to retreat and prompting an investigation by the U.S. Border Patrol that has focused on the intruders' identity. No shots were exchanged and no one was injured in the incident, which occurred shortly after 11 p.m. on Wednesday. The National Guard troops were members of an entry-identification team, assigned to monitor major illegal-alien and drug-smuggling corridors. After forcing the soldiers to flee, the unidentified men retreated into Mexico. National Guard Sgt. Edward Balaban said the troops did not know how many...