The twenty year old Bajirao was appointed Peshwa in succession to his father by Chhatrapati Shahu. By the time of Baji Rao's appointment, Mughal Emperor Muhammad Shah had in 1719 recognized Marathas' rights over the territories possessed by Shivaji at his death.The treaty also included the Maratha rights to collect taxes (chauth or chauthaii and sardeshmukhi) in the six provinces of Deccan.Bajirao believed that the Mughal Empire was in decline and wanted to take advantage of this situation with aggressive expansion in north India. Sensing the declining fortune of the Mughals, he is reported to have said, "Strike, strike at the roots and the biggest tree will also fall down." However, as a new Peshwa, he faced several challenges:These were
Bajirao would often accompany his father on military campaigns. He was with his father when the latter was imprisoned by Damaji Thorat before being released for a ransom. When Vishwanath died in 1720, Shahu appointed the 20-year old Baji Rao as the Peshwa. He is said to have preached the ideal of Hindu Pad Padshahi (Hindu Empire),
On January 4, 1721, Baji Rao met Nizam-ul-Mulk Asaf Jah I at Chikhalthan to settle their disputes through agreement. However, Nizam refused to recognize the Maratha rights to collect taxes from the Deccan provinces. Nizam was made Vizier of Mughal Empire in 1721 , but alarmed at his growing power, Emperor Muhammad Shah transferred him from Deccan to Awadh in 1723. Nizam rebelled against the order, resigned as the Vizier and marched towards Deccan. The Emperor sent an army against him, which the Nizam defeated in the Battle of Sakhar-kheda. In response, Mughal Emperor was forced to recognize him as the viceroy of Deccan. The Marathas, led by Bajirao, helped Nizam win this battle. In fact, for his bravery in the battle, Baji Rao was honored with a robe, a mansabdari of 7,000, an elephant and a jewel. After the battle, Nizam tried to appease both the Maratha Chhatrapati Shahu as well as the Mughal Emperor. However, in reality, he wanted to carve out a sovereign kingdom and considered the Marathas his rivals in the Deccan.
In 1723, Baji Rao had organized an expedition to the southern parts of Malwa. The Maratha chiefs such as Ranoji Shinde, Malhar Rao Holkar, Udaji Rao Pawar, Tukoji Rao Pawar and Jivaji Rao Pawar had successfully collected chauth from several areas in Malwa.(Later, these chiefs carved out their own kingdoms of Gwalior, Indore, Dhar and Dewas States- Junior and Senior respectively). To counter the Maratha influence, Mughal Emperor had appointed Girdhar Bahadur as the Governor of Malwa.
In 1725, Nizam sent an army to clear out the Maratha revenue Collectors from the Carnatic region. The Marathas dispatched a force under Fateh Singh Bhosle to counter him; Baji Rao accompanied Bhosle, but did not command the army. The Marathas were forced to retreat. They launched a second campaign after the monsoon season, but once again, they were unable to prevent the Nizam from ousting the Maratha Collectors.
On August 27, 1727, Baji Rao started a march against Nizam. He raided and plundered several of Nizam's territories, such as Jalna, Burhanpur and Khandesh. While Bajirao was away, Nizam invaded Pune, where he installed Sambhaji II as Chhatrapati. He then marched out of the city, leaving behind a contingent headed by Fazal Beg. On February 28, 1728, the armies of Bajirao and Nizam faced each other at the Battle of Palkhed. Nizam was defeated and forced to make peace. On March 6, he signed the Treaty of Mungi Shevgaon, recognizing Shahu as the Chhatrapati as well as the Maratha right to collect taxes in Deccan.
In Bundelkhand, Chhatrasal had rebelled against the Mughal empire and established an independent kingdom. In December 1728, a Mughal force led by Muhammad Khan Bangash defeated him and imprisoned his family. Chhatrasal had repeatedly sought Bajirao's assistance, but the latter was busy in Malwa at that time. In March 1729, Peshwa finally responded to Chhatrasal's request and marched towards Bundelkhand. Chhatrasal also escaped his captivity and joined the Maratha force. After they marched to Jaitpur, Bangash was forced to leave Bundelkhand. Chhatrasal's position as the ruler of Bundelkhand was restored. Chhatrasal assigned a large jagir to Baji Rao and also married his daughter Mastani to him. Before his death in December 1731, he ceded some of his territories to the Marathas.
After consolidating Maratha influence in central India, Peshwa Baji Rao decided to assert Maratha rights to collect taxes from the rich province of Gujarat. In 1730, he sent a Maratha force under Chimaji Appa to Gujarat. Sarbuland Khan, the Mughal Governor of the province, ceded to Marathas, the right to collect chauth and sardeshmukhi from Gujarat. He was soon replaced by Abhay Singh, who also recognized the Maratha rights to collect taxes. However, this success irked Chhatrapati Shahu's senapati(commander-in-chief) Trimbak Rao Dabhade. His ancestors from Dabhade clan had raided Gujarat several times, asserting their rights to collect taxes from that province. Annoyed at Bajirao's control over what he considered his family's sphere of influence, he rebelled against the Peshwa. Two other Maratha nobles of Gujarat — Gaekwad and Kadam Bande — also sided with Dabhade.
Meanwhile, after the defeat of Girdhar Bahadur in 1728, the Mughal Emperor had appointed Jai Singh II to subdue the Marathas. However, Jai Singh recommended a peaceful agreement with the Marathas. The Emperor disagreed and replaced him with Muhammad Khan Bangash. Bangash formed an alliance with the Nizam, Trimbak Rao and Sambhaji II. On April 1, 1731, Baji Rao defeated the allied forces of Dabhade, Gaekwad and Kadam Bande: Trimbak Rao was killed in the Battle of Dabhoi. On April 13, Baji Rao resolved the dispute with Sambhaji II by signing the Treaty of Warna, which demarcated the territories of Chhatrapati Shahu and Sambhaji II. Subsequently, the Nizam met Baji Rao at Rohe-Rameshwar on December 27, 1732 and promised not to interfere with the Maratha expeditions.
The Siddis of Janjira controlled a small but strategically important territory on the western coast of India. They originally held only the Janjira fort, but after Shivaji's death, they had expanded their rule to a large part of the central and northern Konkan region. After the death of the Siddi chief Yakut Khan in 1733, a war of succession broke out among his sons. One of his sons, Abdul Rehman, requested Baji Rao for help. Baji Rao sent a Maratha force led by Sekhoji Angre(son of Kanhoji Angre). The Marathas regained control of several places in Konkan and besieged Janjira. However, their strength was diverted after Peshwa's rival Pant Pratinidhi occupied the Raigad Fort near Janjira in June 1733. In August, Sekhoji Angre died, further weakening the Maratha position. As a result, Baji Rao decided to sign a peace treaty with the Siddis. He allowed the Siddis to retain control of Janjira on the condition that they would accept Abdul Rehman as the ruler. The Siddis were also allowed to retain control of Anjanvel, Gowalkot and Underi. The Marathas retained the territories of Raigad, Rewas, Thal and Chaul, which they had gained during the offensive.
Though Baji Rao was essentially monogamous by both, nature and family tradition, he took a second wife Mastani. She was the daughter of the Hindu king Chhatrasal of Bundelkhand from his Muslim concubine. The marriage was purely a political one and was accepted out of regard for the sentiments of the Bundela king. In 1734, Mastani bore a son who was to be named Krishna Rao at birth. Being born of a Muslim mother, the Priests refused to conduct the Hindu upanayana ceremony for him.. The boy was eventually named Shamsher Bahadur and brought up as a Muslim.
On November 12, 1736, the Peshwa started a march to the Mughal capital Delhi from Pune. On hearing about the advancing Maratha army, Mughal Emperor asked Saadat Ali Khan I to march from Agra and check the Maratha advance. The Maratha chiefs Malhar Rao Holkar and Pilaji Jadhav crossed Yamuna and plundered the Mughal territories in the Ganga-Yamuna Doab. Saadat Khan led a force of 150,000 against them, and defeated them. He then retired to Mathura, thinking that the Marathas had retreated. However, Baji Rao advanced to Delhi and encamped at Talkatora. The Mughal Emperor dispatched a force led by Mir Hasan Khan Koka to check his advance. The Marathas defeated this force in the Battle of Delhi on March 28, 1737. Baji Rao then retreated from Delhi, apprehensive about the approach of a larger Mughal force from Mathura.
The Mughal Emperor Muhammad Shah then sought help from Nizam. Nizam set out from Deccan and met Baji Rao's returning force at Sironj. Nizam told Baji Rao that he was going to Delhi to repair his relationship with the Mughal Emperor. On reaching Delhi, he was joined by other Mughal chiefs and a massive Mughal army set out against the Peshwa. Peshwa also assembled a force of 80,000 Soldiers and marched towards Delhi, leaving behind a force of 10,000 under Chimnaji to guard Deccan. The two armies met mid-way at Bhopal, where the Marathas defeated the Mughals in the Battle of Bhopal on December 24, 1737. Once again, Nizam was forced to sign a peace agreement, this time at Doraha on January 7, 1738. The province of Malwa was formally ceded to the Marathas and the Mughals agreed to pay ₹ 5,000,000 as indemnity. This time, Nizam took an oath on Koran to abide by the treaty.
The Portuguese had captured several territories on the west coast of India. They had violated an agreement to give the Marathas a site on Salsette Island for building a factory and had been practising religious intolerance against Hindus in their territory. In March 1737, Peshwa dispatched a Maratha force led by Chimaji against them. Marathas captured the Ghodbunder Fort and almost all of Vasai, after the Battle of Vasai. They also managed to gain control of Salsette on May 16, 1739, after a prolonged siege. However, the Marathas had to turn their attention away from the Portuguese due to Nader Shah's invasion of the Mughal Empire in the north.
Baji Rao died on 28 April 1740, at the age of 39 of a sudden fever, possibly heat stroke, while inspecting his jahgirs. At that time, he was en route Delhi with nearly 100,000 troops under his command at his camp in the Khargone district, near the city of Indore. He was cremated on the same day at Raverkhedi on Narmada River.
Baji Rao was renowned for rapid tactical movements in battle, using his cavalry inherited from Maratha generals including Santaji Ghorpade, Dhanaji Jadhav and Ananatrao Makaji. Field-Marshal Bernard Montgomery, in his "History of Warfare" likened Bajirao's approach to that subsequently made famous by U.S. Civil War General william Tecumseh Sherman during his 1864 March to the Sea: the use of rapid movements where his troops lived off the land, with minimal concern for their own supply and communication lines and employing "total warfare" on the enemy civilian population. He is often called a cavalry general. Two examples are the Battle of Palkhed in 1728 when he outmaneuvered the Mughal Governor of Deccan province and again in the battle against the Mughal Emperor, Muhammad Shah at Delhi during 1739. British General Montgomery called Bajirao's victory at Palkhed as a "masterpiece of strategic mobility".