MARINE CORPS AIR STATION YUMA, Ariz. -- Marines preparing for combat in Iraq or Afghanistan have lost significant amounts of training time because undocumented immigrants from Mexico have constantly wandered onto a bombing test range in Arizona, according to the commander of this base along the border.

Virtually every Marine squadron headed to Iraq or Afghanistan receives combat training at the Marine Corps Air Station in Yuma, which for nearly 40 miles touches the US-Mexico border in the southwestern corner of Arizona. The Border Patrol's focus in recent years on tightening the border in the eastern part of the state, where volunteer citizens this month have established their own observation posts, has pushed more undocumented immigrants westward.

Since July 2004, the training range has been shut down more than 500 times because of immigrants spotted on the range, causing a loss of more than 1,100 training hours, said Colonel James J. Cooney, the base's commanding officer...