<P> With the help of the Scots, Parliament won at Marston Moor (2 July 1644), gaining York and the north of England . Cromwell's conduct in this battle proved decisive, and demonstrated his potential as both a political and an important military leader . The defeat at the Battle of Lostwithiel in Cornwall, however, marked a serious reverse for Parliament in the south - west of England . Subsequent fighting around Newbury (27 October 1644), though tactically indecisive, strategically gave another check to Parliament . </P> <P> In 1645 Parliament reaffirmed its determination to fight the war to a finish . It passed the Self - denying Ordinance, by which all members of either House of Parliament laid down their commands, and re-organized its main forces into the New Model Army, under the command of Sir Thomas Fairfax, with Cromwell as his second - in - command and Lieutenant - General of Horse . In two decisive engagements--the Battle of Naseby on 14 June and the Battle of Langport on 10 July--the Parliamentarians effectively destroyed Charles' armies . </P> <P> In the remains of his English realm Charles attempted to recover a stable base of support by consolidating the Midlands . He began to form an axis between Oxford and Newark on Trent in Nottinghamshire . Those towns had become fortresses and showed more reliable loyalty to him than to others . He took Leicester, which lies between them, but found his resources exhausted . Having little opportunity to replenish them, in May 1646 he sought shelter with a Presbyterian Scottish army at Southwell in Nottinghamshire . Charles was eventually handed over to the English Parliament by the Scots and was imprisoned . This marked the end of the First English Civil War . </P> <P> Charles I took advantage of the deflection of attention away from himself to negotiate a secret treaty with the Scots, again promising church reform, on 28 December 1647 . Under the agreement, called the "Engagement", the Scots undertook to invade England on Charles' behalf and restore him to the throne on condition of the establishment of Presbyterianism for three years . </P>

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