a face-to-face encounter. English cavalry tactics were intended to utilise their strengths. They would advance in a tight formation, their riders' legs interlocked, at no faster than a trotto maintain formation. They would discharge their pistols at very short range and upon coming into contact attempt to use the sheer weight of their mounts and the mass of their formation to force back their opponents and burst through their ranks. Both armies contained dragoons. These had originated as mounted infantry, using horses to increase their operational mobility and dismounting to fight with pikes or muskets. By 1650 they had largely become specialist mounted troops; none carried pikes. The English dragoons had exchanged their muskets for carbines (shorter-barrelled versions of the infantry's muskets) or, occasionally, pistols and been formally recognised as a cavalry arm. Scottish dragoons were part way through this transformation and carried both matchlock muskets and cavalry swords. Dragoons usually acted as scouts, or formed their army's rearguard. Prelude English invasion of Scotland Scotland was actively rearming and the leaders of the English Commonwealth felt threatened. They pressured Thomas Fairfax, lord general of the New Model Army, which at this point was synonymous with the Parliamentarian army, to launch a preemptive attack. Fairfax was unwilling to strike the first blow against his former allies, believing England and Scotland were still bound by the Solemn League and Covenant. Oliver Cromwell succeeded him as commander-in-chief of the New Model Army and led it across the Tweed into Scotland on 22 July 1650 and so starting the Third English Civil War. Once the Treaty of Breda had been signed the Scottish Parliament started levying men to form a new army, under the command of the experienced general David Leslie. By the time Cromwell entered Scotland Leslie had some 8,000–9,500 infantry and 2,000–3,000 cavalry, although these numbers fluctuated during the course of the campaign. The government instituted a commission to purge the army of anyone suspected of having supported the Engagement, as well as men considered sinful or undesirable. This was opposed, unsuccessfully, by much of the Scottish nobility and the more experienced military leaders, including Leslie. The purge removed many experienced men and officers and the bulk of the army was composed of raw recruits with little training or experience. Leslie prepared a defensive line of earthworks between the Scottish capital Edinburgh and Leith, employed a scorched earth policy from there to