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Descent upon the Carnatic

The army Hyder assembled was one of the largest seen in southern India, estimated to number 83,000. Carefully co-ordinating the actions of his subordinate commanders, he swept down the Eastern Ghats onto the coastal plain in July 1780, laying waste the countryside. Due to Hyder's secrecy and poor British intelligence, officials in Madras were unaware of his movements until the fires of burning villages just 9 miles (14 km) away were seen in Madras. Hyder himself organised the Siege of Arcot, while detaching his son Karim Khan Sahib to take Porto Novo. The movement in August of Sir Hector Munro with a force of over 5,000 from Madras to Kanchipuram (Conjeevaram) prompted Hyder to lift the siege of Arcot and move to confront him. Word then arrived that Munro was awaiting the arrival of reinforcements from Guntur under Colonel William Baillie, so he sent a detachment under Tipu to intercept them, and eventually followed in strength himself, when Munro sent a force from his army to meet Baillie. Tipu and Hyder surrounded Baillie's force, and compelled the surrender of about 3,000 men in the Battle of Pollilur on 10 September; it was the first effective use of rocket artillery and made a strong impression upon the British. Hyder then renewed the siege of Arcot, which fell in November.

Shortly after the outbreak of hostilities, Governor Hastings had sent General Sir Eyre Coote south from Bengal to take charge of British forces opposing Hyder. He arrived at Madras in November to take command from Munro. Coote marched into the Carnatic, and eventually occupied Cuddalore. After being re-supplied there, he besieged Chidambram, where an assault on the fort was repulsed.

Hyder had in the mean-time descended into Tanjore, with severe consequences. After extracting the allegiance of the Maratha king Thuljaji, Hyder plundered the country, destroying cattle and crops. The economic output of Tanjore is estimated to have fallen by 90% between 1780 and 1782. Hyder's ravages were followed by alleged expeditions of plunder launched by the Kallars. The economic devastation wrought by these attacks was so severe that Tanjore's economy did not recover until the start of the 19th century; the era is referred to in local folklore as the Hyderakalam.

With General Coote at Cuddalore, Hyder then made a forced march to interpose his army between Chidambram and Cuddalore, cutting Coote's supply line. Coote marched to face him, and won a decisive victory in the Battle of Porto Novo on 1 July 1781; Coote estimated that Hyder lost 10,000 men in the battle. Hyder then dispatched Tipu in an attempt to prevent the junction of Coote's army with reinforcements from Bengal. This failed, and in late August the two armies met again at Pollilur, chosen by Hyder as a place to make a stand, because it was the site of his victory over Baillie the previous year. Hyder was defeated this time, although the battle was not decisive. While Coote re-grouped and searched for provisions, Hyder took the opportunity to besiege Vellore. Madras authorities convinced the ageing Coote to put off his retirement and relieve the fortress there. Hyder and Coote met in battle at Sholinghur, near Vellore. Hyder's artillery was ineffective, and the re-provisioned Vellore, which had been on the brink of surrender.

Lord Macartney, who had recently arrived to take the Governorship of Madras, also brought news that Britain was at war with the Dutch. Consequent to this, the company was instructed to seize Dutch holdings in India, and Macartney had ordered a detachment from Tanjore, under Colonel Braithwaite, to capture the main Dutch post at Negapatam. Hyder made an agreement with the Dutch to provide troops for its defence, but was himself forced away from Negapatam by Braithwaite. The British took Negapatam after a three-week siege in October and November 1781. This setback forced Hyder to withdraw from most of Tanjore.

In January 1782, General Coote, his health failing, again set out to re-provision Vellore. Hyder did not prevent the re-supply, but shadowed the British back toward Tripassore, offering battle near Sholinghur. Coote successfully manoeuvred away from Hyder without battle. In February, Hyder detached Tipu with a sizeable force to recover Tanjore. Intelligence failures led the main British garrison to become surrounded by this superior force; Colonel Brathwaite and 2,000 men surrendered. Hyder was also pre-occupied by bad news from the west. A Mysorean force that had been besieging Tellicherry was broken, with its commander and his siege guns captured, and Coorg and Malabar were also descending into open rebellion. Hyder consequently sent forces west to deal with these matters, and was preparing to follow himself when word arrived on 10 March that a French force had landed at Porto Novo. Hyder immediately sent Tipu from Tanjore to meet with them, and followed himself from Arcot. At this time he had a celebrated meeting with the French Admiral Suffren, and the allies agreed on a plan to establish Cuddalore as a French base. Cuddalore was occupied without resistance on 8 April, and Hyder's army, joined by the French, marched toward Permacoil, which fell in May. Coote responded by marching toward Arni, where Hyder had a major supply depot. Hyder and the French had been considering an assault on Wandiwash, but abandoned that idea and marched to face Coote. They skirmished there on 2 June. In August the British landed a force on the Malabar coast, to which Hyder responded by sending additional troops under Tipu to the west. The onset of the monsoon season then suspended most military activity on the eastern plain, and Hyder established his camp near Chittoor.