After the departure of Nadir Shah, Muhammad Shah could not recognize his empire. Marathas were gaining strength. So, Muhammad Shah's domain was confined to central areas.
The Battle of Karnal (24 February 1739),[11] was a decisive victory for Nader Shah of Iran, during his invasion of Mughal dynasty of India. Nader's forces defeated the army of Muhammad Shah within three hours, despite being heavily outnumbered (six-to-one),[12] paving the way for the Persian sack of Delhi. The engagement is considered the crowning jewel in Nader's military career as well as a tactical masterpiece.[7][13][14] The battle took place near Karnal, 110 kilometres (68 mi) north of Delhi, India.[1]
The battle was the culminating point of Nader Shah's invasion of the Mughal Empire. After his conquest of eastern Afghanistan and invasion via Kabul and Peshawar, Nader led his forces south towards the Mughal capital. At Delhi Muhammad Shah gathered an extremely large force with which he marched north before his cumbersome army ground to a halt at Karnal. Nader gave battle and won a crushing victory. In the negotiations following the catastrophic defeat, Muhammad Shah agreed to pay a large indemnity in exchange for maintaining his imperium over his lands. Nader however, forced the Mughal emperor to submit utterly and marched him to his capital, Delhi, where the Mughal treasury was plundered. An uprising against Nader's soldiers by Delhi's citizens ended in a bloody massacre where the entire city was sacked and looted. The enormous plunder gained in Delhi caused Nader to issue an imperial decree removing all taxes for a total of three years. The Persian army soon after departed leaving behind 30,000 dead. Muhammad Shah was also forced to concede all his lands west of the Indus which were annexed by Nader Shah.
As a result of the overwhelming defeat of the Mughal Empire at Karnal, the already declining Mughal dynasty was critically weakened to such an extent as to hasten its demise. It is also possible that without the ruinous effects of Nader's invasion of India, European colonial takeover of the Indian subcontinent would have come in a different form or perhaps not at all
Nader's casus belli
A popular destination for the plethora of Afghan mercenaries and warlords who were defeated in Nader's numerous campaigns against the various polities in Afghanistan, were the northern border areas of the Mughal Empire. Nader had sent a number of requests to the local governors and statesman of northern India requesting that these fugitives be captured and handed over to him. In the aftermath of his conquest of Afghanistan Nader was seeking a pretext for invading the Mughal Empire. Naturally he seized upon this opportunity to mask his invasion in the form of a hunt for the Afghan warriors who had found asylum in the rugged terrain of the northern Mughal realm.[15]
It is also noteworthy that Nader had been in contact with one of the main Mughal ministers, Nizam-ul-Mulk, who some at the time accused of treachery against the Mughal emperor. Whilst it is possible that Nizam-ul-Mulk did in fact provoke Nader's invasion of the Mughal Empire, it remains an implausible conjecture.
Whether or not the Mughal Empire could have defused the situation diplomatically is still a matter of discussion. The ability of the Mughal state to find and imprison Afghan fugitives in its border regions is a seriously doubtful issue as is the actual number of fugitives who in fact escaped to these regions to begin with. It is very possible that they were an insubstantial number. In any case, the intention of Nader to invade Mughal India would have been the same regardless of whether or not the Mughal authorities granted his request for seeking out and handing over fugitives since Nader was merely utilising this as a pretext