A Mexican army convoy began crossing into the United States on Thursday to bring aid to victims of Hurricane Katrina (search).

Carrying water treatment plants and mobile kitchens that can feed 7,000 people daily, the convoy bound for San Antonio is the first Mexican military unit to operate on U.S. soil since 1846.

The first of 45 vehicles in the convoy crossed the international bridge at Laredo at about 8:15 a.m. Military engineers, doctors and nurses are among the 200 people headed to San Antonio.

Mexico has sent disaster relief aid missions to other Latin American nations, but not to the United States.

In 1846, Mexican troops briefly advanced just north of the Rio Grande (search) in Texas, which had then recently joined the United States. Mexico, however, did not then recognize the Rio Grande as the U.S. border.

The two countries quickly became mired in the Mexican-American War (search), which led to the loss of half of Mexico's territory in 1848.