Complex sentence: In 1753, he was an ensign in the 20th Regiment of Foot and saw action with his regiment during the Seven Years' War and as an observer during the Russo-Turkish War of 1768-1774.
Split into simpler sentences: In 1753, he was an ensign in the 20th Regiment of Foot and saw action with his regiment during the Seven Years' War. After the conculusion of the Seven years War Thomas Carleton served as an observer during the Russo-Turkish War of 1768-1774.