Jackson began his United States Army career as a second lieutenant in Company K of the 1st U.S. Artillery Regiment, proceeding through Pennsylvania, down the Ohio and Mississippi Rivers to New Orleans, and from there the troops embarked for Point Isabel, Texas, and were sent to fight in the Mexican–American War from 1846 to 1848. Jackson's command was directed to report to General Taylor and proceed immediately via Matamoros and Camargo to Monterey and then to Saltillo. Prior to the Battle of Buena Vista, Lieutenant Jackson's command was ordered to withdraw from General Taylor's army and march to the mouth of the Rio Grande, where they would be transferred to Veracruz. He served at the Siege of Veracruz and the battles of Contreras, Chapultepec, and Mexico City, eventually earning two brevet promotions, and the regular army rank of first lieutenant. It was in Mexico that Thomas Jackson first met Robert E. Lee.