These reserve regiments were composed of Kurds, rural Turks and a few Armenians. The performance of the reserve divisions was poor, and in March 1915 the forces that survived were turned into two divisions totalling only two thousand men and seventy officers. Later that month, the best regiments were consolidated into one division and the rest disbanded. Nonetheless, cavalry was used by Ottoman forces throughout 1915 in engagements with the Russians, and one cavalry unit even exchanged small arms fire with a submarine crew in the Dardanelles in early 1915. Turkish cavalry was used in engagements with the British, including the Third Battle of Gaza in late 1917. In this battle, both sides used cavalry forces as strategic parts of their armies. Cavalry continued to be involved in engagements well into 1918, including in conflicts near the Jordan River in April and May that year, which the Ottomans called the First and Second Battles of Jordan, part of the lead-up to the Battle of Megiddo. By September 1918, regular army cavalry forces were stationed throughout the Turkish front, and the only remaining operationally ready reserve forces in the Ottoman military were two cavalry divisions, one formed after the initial problems in 1915. By 1916, the United States Cavalry consisted of 15,424 members organized into 15 regiments, including headquarters, supply, machine-gun and rifle troops. Just before formally joining the war effort, the US had gained significant experience in 1916 and 1917 during the Pancho Villa Expedition in Mexico, which helped to prepare the US Cavalry for entry into World War I. In May 1917, a month after the US declaration of war, the National Defense Act went into effect, creating the 18th through the 25th US Cavalry regiments, and later that month, twenty more cavalry regiments were created. However, British experiences during the first years of the war showed that trench warfare and weapons that included machine guns and artillery made cavalry warfare impractical. Thus, on October 1, eight of the new cavalry regiments were converted to field artillery regiments by order of Congress, and by August 1918, twenty National Army horse units were converted to thirty-nine trench mortar and artillery batteries. Some horse units of the 2nd, 3rd, 6th and 15th Cavalry regiments accompanied the US forces in Europe. The soldiers worked mainly as grooms and farriers, attending to remounts for the artillery, medical corps and transport services. It was not until late August 1918 that US cavalry entered combat. A provisional squadron of 418 officers and enlisted men, representing the 2nd Cavalry Regiment, and mounted on convalescent horses, was created to serve as scouts and couriers during the St. Mihiel Offensive.