The Battle of Karnal on February 24, 1739 all but sealed the eventual fate of the once mighty Mughal Empire.The army of Nadir Shah, founder of the Afsharid dynasty of Iran, defeated the forces of Mughal emperor Muhammad Shah ‘Rangila’ in under three hours. The Shah of Iran subsequently captured and sacked Delhi, the Mughal capital, and emptied the royal treasury, taking home the fabled Peacock Throne and the Koh-i-noor diamond embedded on it.While Nadir Shah spared Rangila’s life, and even restored most of his territory, he left behind a permanently enfeebled Mughal Empire. The Mughals would rule Delhi for another 118 years but with progressively diminishing power, so much so that the authority of the last Mughal Emperor Bahadur Shah ‘Zafar’ hardly extended beyond the four walls of the Red Fort.Here’s a brief history of how this decline started, and the battle which sealed the Mughal Empire’s fate.Mughal decline“The invasion of Nadir Shah… [was] not a cause of the decline of the Mughal Empire, but one of the clearest symptoms of the decline,” historian Jadunath Sarkar said in 1922 at the Patna University. (His lectures at the university would later be put together in the book Nadir Shah in India first published in 1925).Although scholars offer differing assessments of what caused the decline of the once-mighty empire, there is general consensus today that there were structural reasons behind it — not just that the emperors who came after Aurangzeb were “weak”.Must Read | At home with Irfan Habib: The History KeeperHistorian Irfan Habib, for instance, argues that the decline was due to the excessive tax burden on the peasantry, which revolted in many areas. Since resources were needed to crush these revolts, the tax burden further went up, leading to more revolts. This created a sort of vicious cycle which was at the heart of the decline of the Mughal Empire, Habib argues. (The Agrarian System of Mughal India, 1963).Story continues below this adOn the other hand, M Athar Ali points to the bloating of the Mughal nobility in the late 17th century. While there was an influx of new nobles, there were not enough “good jagirs (land revenue assignments) to support them” which led to corruption, infighting, and a decline in military performance. (The Mughal Nobility under Aurangzeb, 1966).Then there are historians of Sarkar, who tend to focus on the alienation of Hindus and other religious minorities due to the oppressive religious policies of Aurangzeb.Nadir Shah’s invasionWhat can be said with certainty is that by the time of Aurangzeb, the Empire faced constant challenges, whether they be from the Marathas in the south, Ahoms in the east, or the Jats, Raputs, Bundelas, and Sikhs in the north and west. These challengers were bleeding the empire dry with a 1,000 cuts, not only taking territory and capturing wealth, but straining the Mughal treasury in the process.The invasion of Nadir Shah would be the single-greatest challenge that the Mughal Empire would face till that date. A military genius who some historians would later refer to as the “Napoleon of Persia”, Nadir Shah took over power in Iran after deposing the reigning Safavid dynasty. He not only consolidated power in Persia but carried out successful military campaigns far beyond, clashing with the Ottomans in the west, Russians in the north, Afghan tribes to his east, and of course, the Mughals.Story continues below this adAfter conquering Kandahar in 1738, Nadir Shah set his sights on India. He crossed into the subcontinent through the famed Khyber pass — the route taken by previous invaders from the west, from Alexander to Timur. After swiftly overcoming a number of Mughal vassal states, Nadir Shah marched towards Delhi.Rangila took time to react to Nadir Shah’s invasion. As Sarkar put it, “the proceedings of the [Delhi] Court during Nadir’s invasion form a tale of disgraceful inefficiency, amounting to imbecility.” Rangila waited until December to spring to action even though Nadir Shah had taken Kabul in June and had crossed the Khyber pass by mid-November. In fact, it was not until January that a Mughal army was ready to meet Nadir Shah’s marauding forces.By this time, the Persians had taken Lahore and were on their way to Delhi. The two armies would meet in Karnal (present-day Haryana), some 125 km away from the capital.The battle & beyondThe Mughal army was 300,000-strong, boasting more than 2,000 war elephants and 3,000 cannons. Counting all non-combatants — Rangila travelled with an expansive harem and massive retinue of servants — the Mughal party might have been a million strong, by some accounts.Story continues below this adAlso Read | How Akbar and Aurangzeb have contrasting images in India and PakistanNadir Shah’s army was much smaller, only 55,000 strong. But it was also more disciplined, significantly more experienced, and boasted more modern tactics and weaponry. This, along with Nadir Shah’s own genius, proved to be no match for the Mughals.“Nader Shah lured [Mughal commander] Sa’adat Khan’s old-fashioned heavy Mughal cavalry into making a massed frontal charge. As they neared the Persian lines, Nader’s light cavalry parted like a curtain, leaving the Mughals facing a long line of mounted musketeers, each of them armed with the latest in eighteenth-century weaponry: armour-penetrating, horse-mounted swivel guns. They fired at point-blank range. Within a few minutes, the flower of Mughal chivalry lay dead on the ground,” historians William Dalrymple and Anita Anand wrote in Koh-i-Noor: The History of the World’s Most Infamous Diamond (2016).The Mughal army was routed in under three hours, and Muhammad Shah was subsequently captured. The Persians then marched to Delhi where they plundered the royal treasure, and massacred thousands of non-combatants in one of the most violent episodes in Delhi’s history.Some 30,000 citizens of Delhi were slaughtered. “The Persians laid violent hands on everything and everybody; cloth, jewels, dishes of gold and silver were acceptable spoil,” wrote historian-administrator Ghulam Hussain Khan (as quoted by Dalrymple and Anand). Thousands of women were enslaved, and whole mohallas around Dariba Kalan were gutted.Story continues below this adAlso Read | ‘Panipat’ controversy: Why Maharaja Surajmal matters in RajasthanNadir Shah eventually handed Rangila his territories back, and even married his son to Rangila’s niece. But he left Delhi with what was the “accumulated wealth of eight generations of imperial Mughal conquest,” Dalrymple and Anand wrote. The Mughal coffers were all but empty, meaning that Delhi could not, even if it wanted to, control the territory that it technically had.Over the next hundred-odd years, the Mughal Empire ceded more and more territory, till the British finally ended Mughal rule once and for all in 1857.