I came across this statement somewhere that Nadir Shah scooted away when he heard that Bajirao is marching North. This means, we are staring at a completely different narrative of the story presented to us, only thing is, we need to find out how much of this story is true.
All this started with the Marathas demanding Chauth and Sardeshmukhi for all Mughal provinces south of Khandesh and Gujarat including them. The Saiyyad Brothers, who were ruling Delhi after making the Emperor Farrukhsiyar their puppet agreed to this but the Emperor refused. Farrukhsiyar tried to depose the Saiyyad Brothers but was betrayed. Well, the brothers and the Peshwa retaliated by sending a Maratha army under Parsoji Bhosale who deposed the Emperor.
This was followed by a lull in the north as the next Peshwa, Bajirao concentrated his energies to crush the Nizam, the Sidis and other Mughal subsidaries in the South and turned his eyes towards Delhi only by 1737. In a lightning campaign, he defeated the Mughals just outside the city of Delhi and his best army, led by the Nizam at Bhopal.
The absolute destruction of Mughal pride shook the world and declared to the world that India is ripe for harvest. At the opportune time, a warlord, Nadir Shah started to gain ground in Persia. We can argue two reasons why Nadir Shah turned his eyes towards India – the Mughal nobles, fearing that the Marathas will topple them, decided to maintain the status quo by using Nadir Shah to crush the Marathas. There were precedents, the best being Babur himself. The other reason may be, a genuine opportunity to loot. Going by the fact that he reached the vicinity of Delhi(Karnal is 100 km from Delhi and 35 km from Panipat) unopposed is a clear indicator of the tacit pact.
Bajirao was busy cutting down the Portuguese to size when Nadir Shah came to India. It is possible that Nadir Shah timed his entry at a time Bajirao can’t field a formidable force against him. However, due to the massacre of Delhi on Holi day in 1738 and subsequent depradations, the Peshwa was forced to turn towards Delhi only by May 1739 after his forces were freed from the Seige of Vasai. Though the Portuguese lost everything, Goa survived because of this. Nadir Shah’s departure exactly coincided with the Maratha march North. This is the reference I was alluding to – Nadir Shah decided to scoot away hearing about the Maratha advance. Bajirao died on 28 April 1740 marching North while Nadir Shah decamped on 1 May 1740.
This is an interesting conjencture. What happens if there was no Portuguese angle and what if Bajirao was there in North trying to stop Nadir Shah? If Baji Rao wins, would he have crowned Shahu the emperor of India? What will happen to the history of India and as a byproduct, to the history of the world? But, the reality is this. By acting as a punching bag to the Marathas, the Portuguese changed the world history forever.