In August 1642, both sides expected the conflict to be settled by a single battle, but it soon became clear this was not the case. Royalist success in 1643 led to Parliament agreeing a military alliance with the Scots Covenanters, and winning a series of battles in 1644, the most significant being the Battle of Marston Moor. The formation of the New Model Army gave Parliament the first professional military force in England, and their success at Naseby in June 1645 proved decisive.