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The Eastern Association of counties was an administrative organisation set up by Parliament in the early years of the First English Civil War. Its main function was to finance and support an army which became a mainstay of the Parliamentarian military effort until early 1645, when many of its units were incorporated into the New Model Army.
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. The Grand Committee of the Eastern Association, and its different sub-committees, sat in the Bear Inn in Cambridge throughout 1643 and 1644. 
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 Covenanters under the Earl of Leven in the Siege of York. On 1 July, the besiegers were outmanoeuvred by Prince Rupert of the Rhine, who skirted them and reached 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. Cromwell's cavalry - aided by Covenanter regiments - first drove off Royalist cavalry on their side of the field. Showing discipline they rallied beyond the Royalist forces and then attacked the Royalist cavalry under Goring on the other side and routed the Royalists from the field. The siege of York was resumed and the city fell to Parliamentarians on 16 July.
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.
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The Battle of Marston Moor was fought on 2 July 1644, during the Wars of the Three Kingdoms of 1639 – 1653. The combined forces of the English Parliamentarians under Lord Fairfax and the Earl of Manchester and the Scottish Covenanters under the Earl of Leven defeated the Royalists commanded by Prince Rupert of the Rhine and the Marquess of Newcastle.
Robert Devereux, 3rd Earl of Essex, KB, PC was an English Parliamentarian and soldier during the first half of the 17th century. With the start of the Civil War in 1642, he became the first Captain-General and Chief Commander of the Parliamentarian army, also known as the Roundheads. However, he was unable and unwilling to score a decisive blow against the Royalist army of King Charles I. He was eventually overshadowed by the ascendancy of Oliver Cromwell and Thomas Fairfax, and resigned his commission in 1646.
The New Model Army was a standing army formed in 1645 by the Parliamentarians during the First English Civil War, then disbanded after the Stuart Restoration in 1660. It differed from other armies employed in the 1639 to 1653 Wars of the Three Kingdoms in that members were liable for service anywhere in the country, rather than being limited to a single area or garrison. To establish a professional officer corps, the army's leaders were prohibited from having seats in either the House of Lords or House of Commons. This was to encourage their separation from the political or religious factions among the Parliamentarians.
The Battle of Nantwich was fought on 25 January 1644 in Cheshire during the First English Civil War. In the battle, Sir Thomas Fairfax in command of a Parliamentarian relief force defeated Lord Byron and the Royalists.
The Self-denying Ordinance was passed by the English Parliament on 3 April 1645. All members of the House of Commons or Lords who were also officers in the Parliamentary army or navy were required to resign one or the other, within 40 days from 3 April 1645.
The Ironsides were troopers in the Parliamentarian cavalry formed by English political leader Oliver Cromwell in the 17th century, during the English Civil War. The name came from "Old Ironsides", one of Cromwell's nicknames.
The Irish Confederate Wars, also called the Eleven Years' War, took place in Ireland between 1641 and 1653. It was the Irish theatre of the Wars of the Three Kingdoms, a series of civil wars in the kingdoms of Ireland, England and Scotland – all ruled by Charles I. The conflict had political, religious and ethnic aspects and was fought over governance, land ownership, religious freedom and religious discrimination. The main issues were whether Irish Catholics or British Protestants held most political power and owned most of the land, and whether Ireland would be a self-governing kingdom under Charles I or subordinate to the parliament in England. It was the most destructive conflict in Irish history and caused 200,000–600,000 deaths from fighting as well as war-related famine and disease.
The First English Civil War took place in England and Wales from 1642 to 1646. It is part of the 1639 to 1653 Wars of the Three Kingdoms, which also include the Bishops' Wars, the Irish Confederate Wars, the Second English Civil War, the Anglo-Scottish war (1650–1652) and the Cromwellian conquest of Ireland. Historians calculate some 15% to 20% of all adult males in England and Wales served in the military between 1639 to 1653, while around 4% of the total population died from war-related causes, versus 2.23% for World War I. These figures illustrate the impact of the conflict on society in general, and the bitterness it engendered.
The Second Battle of Newbury was a battle of the First English Civil War fought on 27 October 1644, in Speen, adjoining Newbury in Berkshire. The battle was fought close to the site of the First Battle of Newbury, which took place in late September the previous year.
The Battle of Winceby took place on 11 October 1643 during the First English Civil War near the village of Winceby, Lincolnshire. In the battle, a Royalist relieving force under the command of Sir William Widdrington was defeated by the Parliamentarian cavalry of the Earl of Manchester.
Sir Edward Rossiter was an English landowner, soldier and politician from Lincolnshire. He fought with the Parliamentarian army in the Wars of the Three Kingdoms and sat as an MP at various times between 1646 and 1660.
The siege of York in 1644 was a prolonged contest for York during the First English Civil War, between the Scottish Covenanter army and the Parliamentarian armies of the Northern Association and Eastern Association, and the Royalist Army under the Marquess of Newcastle. It lasted from 22 April until 1 July when the city was relieved by Prince Rupert of the Rhine. Rupert and Newcastle were defeated the next day at the decisive Battle of Marston Moor, and the siege resumed until the city was surrendered on easy terms on 16 July.
The Battle of Gainsborough was a battle in the First English Civil War, fought on 28 July 1643. The strategically important town of Gainsborough, Lincolnshire was a Royalist base used for harassing the Parliamentarians who were generally dominant in Lincolnshire, but was taken by Parliamentarians in July 1643. An attempt to recapture Gainsborough by Charles Cavendish and the Royalists was foiled in a battle in which Colonel Oliver Cromwell distinguished himself as a cavalry leader.
Lawrence Crawford was a Scottish soldier who fought in English or other armies on the continent of Europe. His motives were not mercenary, as he fought only for Presbyterian principles or causes.
Cornwall played a significant role in the English Civil War, being a Royalist enclave in the generally Parliamentarian south-west.
The unsuccessful second Siege of Hull by the Royalist Earl of Newcastle in 1643 was a victory for Parliament at the high point of the Royalist campaign in the First English Civil War. It led to the abandonment of the Earl of Newcastle's campaign in Lincolnshire and the re-establishment of Parliament's presence in Yorkshire.
1643 was the second year of the First English Civil War. Politically, the latter months of the year were the turning-point of the war. The King made a truce with the Irish rebels on 15 September which united against him nearly every class in Protestant England. Only ten days after the "Irish Cessation," Parliament at Westminster swore to the Solemn League and Covenant, and the die was cast.
1644 was the third year of the First English Civil War. The King's position continued to decline and the Long Parliament sent the Propositions of Uxbridge, an attempt to end the war, to the king at Oxford
Colonel Nathaniel Rich was a member of the landed gentry from Essex, who sided with Parliament during the Wars of the Three Kingdoms and was "an example of those pious Puritan gentlemen who were inspired by the ideals of the English Revolution". Appointed a colonel in the New Model Army in 1645, then elected MP for Cirencester in 1648, he was a close associate of Oliver Cromwell until the two fell out due to his association with the Fifth Monarchists, a radical religious group that opposed the latter's appointment as Lord Protector in 1653.
The siege of Lincoln took place from 3 to 6 May 1644 during the First English Civil War, when the important town of Lincoln was besieged by Parliamentarian forces under the Earl of Manchester. On the first day, the Parliamentarians took the lower town. The Royalist defenders retreated into the stronger fortifications of the upper town, which encompassed and incorporated Lincoln Castle and Lincoln Cathedral. The siege ended four days later when the Parliamentarian soldiers stormed the castle, taking prisoner the Royalist governor, Sir Francis Fane, and what remained of his garrison.