a provisional division of three brigades (four regiments of cavalry, two of infantry, and 6,600 men), the expedition crossed the border into Mexico to search for Villa, marching in two columns from Columbus and Culberson's Ranch. The 2nd Provisional Cavalry Brigade reached Colonia Dublán after dark on March 17, where Pershing established the main base of operations for the campaign. The 1st Aero Squadron, included in the expedition for liaison duties and aerial reconnaissance on the orders of United States Secretary of War Newton D. Baker, departed San Antonio, Texas, on March 13 by rail with eight Curtiss JN3 airplanes and flew the first aerial reconnaissance of the area from Columbus on March 16, the day after it arrived. The entire squadron flew to the advanced camp at Colonia Dublán on March 19–20, losing two aircraft in the process. Pershing immediately sent the 7th Cavalry (seven troops in two squadrons) south just after midnight on March 18 to begin the pursuit, followed by the 10th Cavalry moving by rail two days later. From March 20 to March 30, as the 11th Cavalry arrived in Columbus by train from Fort Oglethorpe, Georgia, and then forced marched into Mexico, Pershing dispatched four additional "flying columns" through the mountainous territory into the gaps between the original three columns. Persistent winter weather through early April, particularly bitterly cold nights at high altitude, made both pursuit and logistics more difficult. An additional regiment of cavalry and two of infantry were added to the expedition in late April, bringing its total size to 4,800 men. Ultimately more than 10,000 men—virtually every available unit of the Regular Army and additional National Guard troops—were committed to the expedition either in Mexico or its supporting units at Columbus. Because of disputes with the Carranza administration over the use of the Mexico North Western Railway to supply Pershing's troops, the United States Army employed trucks to convoy supplies to the encampment where the Signal Corps also set up wireless telegraph service from the border to Pershing's headquarters. This was the first use of truck convoys in a U.S. military operation and provided useful experience for World War I. During this phase of the campaign Pershing maintained a small mobile headquarters of 30 men using a Dodge touring car for personal transportation, to keep abreast of the moving columns and control their movements, employing aircraft of the 1st Aero Squadron as