From A History of Iran: Empire of the Mind, by Michael Axworthy (Basic Books, 2016), Kindle Loc. 2997-3027:
Crowned shah, with his western frontiers secure and in undisputed control of the central lands of Persia, Nader set off eastward to conquer Kandahar. The exactions to pay for this new campaign caused great suffering and in many parts of the country brought the economy almost to a standstill. Nader took Kandahar after a long siege, but he did not stop there. Using the excuse that the Moghul authorities had given refuge to Afghan fugitives, Nader crossed the old frontier between the Persian and Moghul empires, took Kabul, and marched on toward Delhi. North of Delhi, at Karnal, the Persian army encountered the army of the Moghul emperor, Mohammad Shah. The Persians were much inferior in number to the Moghul forces, yet thanks to the better training and firepower of his soldiers, and rivalry and disunity among the Moghul commanders, Nader defeated them. He was helped by the fact that the Moghul commanders were mounted on elephants, which besides proving vulnerable to firearms were liable to run wild—to the dismay of their distinguished riders and anyone who happened to be in their path.
From the battlefield of Karnal, Nader went on to Delhi, where he arrived in March 1739. Shortly after his arrival there, rioting broke out and some Persian soldiers were killed. So far from home, and with the wealth of the Moghul Empire at stake, Nader could not afford to lose control. He ordered a ruthless massacre in which an estimated thirty thousand people died, mostly innocent civilians. Prior to this point, Nader had generally (at least away from the battlefield) achieved his ends without excessive bloodshed. But after Delhi, he may have decided that his previous scruples had become redundant.
With a characteristic blend of threat and diplomacy, Nader stripped the Moghul emperor of a vast treasure of jewels, gold, and silver, and accepted the gift of all the Moghul territories west of the Indus River. The treasure was worth as much as perhaps 700 million rupees. To put this sum in some kind of context, it has been calculated that the total cost to the French government of the Seven Years’ War (1756–1763), including subsidies paid to the Austrian government as well as all the costs of the fighting on land and sea, was about 1.8 billion livres tournois (the standard unit of account in prerevolutionary France). This was equivalent to about £90 million sterling at the time—close to the rough estimate of £87.5 million sterling for the value of Nader’s haul from Delhi. Some of the jewels he took away—the largest, most impressive ones, like the Kuh-e Nur, the Darya-ye Nur, and the Taj-e Mah—had a complex and often bloody history of their own in the following decades.
Nader did not attempt to annex the Moghul Empire outright. His purpose in conquering Delhi had been to secure the cash necessary to continue his wars of conquest in the west, for which the wealth of Persia alone had, by the time of his coronation, begun to prove inadequate.
Nader’s campaigns are a reminder of the centrality of Persia to events in the region, in ways that have parallels today. A list of some of Nader’s sieges—Baghdad, Basra, Kirkuk, Mosul, Kandahar, Herat, Kabul—has a familiar ring to it after the events of the first years of the twenty-first century. It is worth recalling that Persians were not strangers in any of the lands in which Nader campaigned. Although he and his Safavid predecessors were of Turkic origin and spoke a Turkic language at court, the cultural influence of Persian was such that the language of the court and administration in Delhi and across northern India was Persian, and diplomatic correspondence from the Ottoman court in Istanbul was normally in Persian, too. Persian hegemony from Delhi to Istanbul would, in some ways, have seemed natural to many of the inhabitants of the region, echoing as it did the Persian character of earlier empires and the pervasive influence of Persian literary, religious, and artistic culture.