The Battle of Dunbar was fought between the English New Model Army, under Oliver Cromwell, and a Scottish army commanded by David Leslie on 3 September 1650 near Dunbar, Scotland. The battle resulted in a decisive victory for the English. It was the first major battle of the 1650 invasion of Scotland.

After Charles I's execution in 1649, the English Rump Parliament established a republican Commonwealth in England. When their erstwhile ally, Scotland, recognised his son Charles II as king of all of Britain on 1 May 1650 and began recruiting an army to support him, the English dispatched the New Model Army, under the command of Cromwell. The army crossed into Scotland on 22 July, with a force of over 16,000 men. The Scots withdrew to Edinburgh, stripping the land of provisions. Cromwell attempted to draw the Scots out into a set piece battle, but they resisted, and Cromwell was unable to break through their defensive line. At the end of August, with his army weakened through disease and lack of food, Cromwell withdrew to the port of Dunbar. The Scottish army followed and took up an unassailable position on Doon Hill, overlooking the town. On 2 September, although many of their most experienced men had been dismissed in religious purges, the Scots advanced towards Dunbar and the English took up positions outside the town.

Before dawn on 3 September the English launched a surprise attack on the Scots, who were poorly prepared. The fighting was restricted to the north-eastern flank with the main contingents of English and Scottish cavalry fighting inconclusively, as did the English and Scottish infantry. Due to the terrain Leslie was unable to reinforce the fighting, while Cromwell used his last reserve to outflank the Scots. The Scottish cavalry broke and routed; the Scottish infantry made a fighting retreat but suffered heavy casualties. Between 300 and 500 Scots were killed, approximately 1,000 wounded and at least 6,000 were taken prisoner from an army of 12,500 or fewer.

After the battle, the Scottish government took refuge in Stirling, where Leslie rallied what remained of his army. The English captured Edinburgh and the strategically important port of Leith. In the summer of 1651 the English crossed the Firth of Forth to land a force in Fife; they defeated the Scots at Inverkeithing and so threatened the northern Scottish strongholds. In response, Leslie and Charles II marched the Scottish army south in an unsuccessful attempt to rally Royalist supporters in England. The Scottish government, left in an untenable situation, surrendered to Cromwell, who then followed the Scots south. At the Battle of Worcester, precisely one year after the Battle of Dunbar, Cromwell crushed the Scottish army, ending the war.

After years of rising tensions, the relationship between the king of England, Charles I, and his English Parliament broke down in armed conflict in 1642, starting the First English Civil War. Charles was also, but separately, king of Scotland. He had gone to war with his Scottish subjects in the Bishops' Wars in 1639 and 1640. These had arisen from the Scots' refusal to accept Charles's attempts to reform the Scottish Kirk to bring it into line with English religious practices. Charles was not successful in these endeavours and the ensuing settlement established the Covenanters' hold on Scottish government, requiring all civil office-holders, parliamentarians and clerics to sign the National Covenant and giving the Scottish Parliament the authority to approve all of the king's councillors in Scotland.

In England, Charles's supporters, the Royalists, were opposed by the combined forces of the English Parliamentarians and the Scots, who in 1643 had formed an alliance bound by the Solemn League and Covenant, in which the English Parliament agreed to reform the English church along similar lines to the Scottish Kirk in return for the Scots' military assistance. After four years of war the Royalists were defeated. With his capital at Oxford under siege, Charles escaped on 27 April 1646, surrendered to the Scots at Southwell on 5 May and was taken to Newcastle, which was in Scottish hands. The Scots and the English Parliament agreed on a peace settlement which they put before the King. Known as the Newcastle Propositions, it would have required all of the King's subjects in Scotland, England and Ireland to sign the Solemn League and Covenant, brought the church in each kingdom into accordance with the Covenant and with Presbyterianism and ceded much of Charles's secular authority as king of England to the English Parliament. The Scots spent some months trying to persuade Charles to agree to these terms, but he refused to do so. Eventually, under pressure from the English to withdraw their forces now the war was over, the Scots handed Charles over to the English parliamentary forces in exchange for a financial settlement and left England on 3 February 1647.

Charles then engaged in separate negotiations with different factions. Presbyterian English Parliamentarians and the Scots wanted him to accept a modified version of the Newcastle Propositions, but in June, Cornet George Joyce of the New Model Army seized Charles, and the army council pressed him to accept the Heads of Proposals, a less demanding set of terms which, crucially, did not require a Presbyterian reformation of the church. He rejected these as well and instead signed an offer known as the Engagement, which had been thrashed out with the Scottish delegation, on 26 December. Charles agreed to confirm the Solemn League and Covenant by act of Parliament in both kingdoms and to accept Presbyterianism in England, but only for a trial period of three years, in return for the Scots' assistance in regaining his throne in England.

When the delegation returned to Edinburgh with the Engagement, the Scots were bitterly divided on whether to accept its terms. Its supporters, who became known as the Engagers, argued that it offered the best chance the Scots would get of acceptance of the Covenant across the three kingdoms and that rejecting it risked pushing Charles to accept the Heads of Proposals. It was opposed by those who believed that to send an army into England on behalf of the King would be to break the Solemn League and Covenant and that it offered no guarantee of a lasting Presbyterian church in England; the Kirk went so far as to issue a declaration on 5 May 1648 that condemned the Engagement as a breach of God's law. After a protracted political struggle, the Engagers gained a majority in the Scottish Parliament, by which time war had again broken out in England between Royalists and Parliamentarians. The Scots sent an army under the command of the Duke of Hamilton into England to fight on behalf of the King in July, but it was heavily defeated at Preston by a force led by Oliver Cromwell. The rout of the Engager army led to further political upheaval in Scotland and the faction opposed to the Engagement was able to regain control of the government, with the assistance of a group of English Parliamentary cavalry led by Cromwell.