The first green tractor-trailers, with Mexican flags attached to the tops of their cabs, crossed the international bridge at Laredo, Texas, at about 8:15 a.m. The rest of the 45-vehicle convoy was in a staging area on the U.S. side in about 15 minutes.

Radio talk shows and newspapers in Mexico buzzed with excitement over news that this country, long on the receiving end of U.S. disaster relief, was sending a hurricane aid convoy north.

The convoy has “a very high symbolic content,” said Javier Oliva, a political scientist at Mexico’s National Autonomous University. “This is a very sensitive subject, for historic and political reasons.”