Eastern Association The Eastern Association of counties was a Parliamentarian organisation during the English Civil War . It provided and administered an army which eventually became a mainstay of the Parliamentarian military effort towards the end of 1644 . Foundation . As part of Parliament ' s efforts to improve the administration of its forces , the Parliamentarian militias of Essex , Hertfordshire , Norfolk , Suffolk , Cambridgeshire were established as the " Eastern Association " on 20 December 1642 . Huntingdonshire and Lincolnshire later joined the Association . The first general designated as commander of the Association ' s forces was William Grey , 1st Baron Grey of Werke . One of the units which first became part of the Association ' s force was a troop of horse ( cavalry ) raised by Captain Oliver Cromwell . The counties which composed the Eastern Association were some of the richest agricultural regions of England , so the Association ' s forces were some of best financed and equipped troops on either side in the early part of the civil war . During the winter of 1642 - 43 , it established Parliamentary control over East Anglia . Development . In August 1643 , Lord Grey was replaced as commander by the Earl of Manchester . Oliver Cromwell was appointed Lieutenant General of the Horse . The two men clashed , especially over Cromwell ' s selection of officers . Manchester wrote , " Colonel Cromwell raising of his regiment makes choice of his officers not such as were soldiers or men of estate , but such as were common men , poor and of mean parentage , only he would give them the title of godly , precious men . . . " Late in 1643 , Lawrence Crawford was appointed Sergeant - Major General of the Foot . He too frequently clashed with Cromwell . For much of 1643 , the Association was engaged in battles against the Northern royalists to secure Lincolnshire for Parliament . The Royalists were defeated at Gainsborough and Winceby . In May 1644 , the Association ' s forces besieged Lincoln , the last Royalist post in Lincolnshire . When it was captured , the Association ' s army was free to join the Parliamentarian army under Lord Fairfax and the Scottish Covenanter s under the Earl of Leven in the Siege of York . On 1 July , the besiegers were outmanoeuvered by Prince Rupert of the Rhine , who relieved the city . The next day however , Rupert chose to engage them even though he was outnumbered . In the resulting Battle of Marston Moor , all of Fairfax ' s army and half the Scots fled . However , Manchester ' s infantry and especially the Eastern Association cavalry under Cromwell stood firm and routed the Royalists from the field . Later in the year , the Eastern Association forces moved into the south of England where the Parliamentarian armies under the Earl of Essex and Sir William Waller had suffered setbacks . After the drawn Second battle of Newbury , Manchester was severely criticised by Cromwell for what was seen to be half - hearted leadership . The Eastern Association army had been carrying much of the burden of the war and on 19 November 1644 , the Association announced that it could no longer bear the cost of maintaining its army . This prompted Parliament to decree the formation of the New Model Army . Four cavalry and four infantry regiments of the Eastern Association army were absorbed into the New Model , and became the pattern on which most of the other units were formed . At the same time , the criticism of the Earls of Manchester and Essex resulted in the Self - denying Ordinance , which placed command of the New Model Army in the hands of professional soldiers . Regiments within the Army of the Eastern Association . Cavalry Infantry Dragoons