Chandragupt was aware of the dissatisfaction of the people of Magadha against the rule of Dhanananda, the Nanda ruler of Magadha. Hence, he decided to take advantage of the situation and overthrow the Nand, dynasty from Magadha.
At first he tried to overthrow Dhananand, with the help of Alexander. Having failed in his mission Chandragupta himself raised an army. Soldiers were recruited from the tribal. Chandragupta also had obtained the support and service of a very intelligent and powerful man named Chanakya.
Thus it was possible for him to overthrow Dhanananda, the last Nanda rule of Magadha, and occupy the throne. Having occupied Magadha; Chandragupta systematically expanded the territories of that Magadha kingdom.
He got possession of the north-waster provinces driving out all the agents of Alexander. Next he obtainec Kabul, Kandahar, Baluchistan and Herat after defeating Seleukus the Greek king of Syria. However, under Chandragupta, the Magadha Empire extended up to present Karnataka region in south India.
In short, entire north, north-western and eastern India, except Kashmir and Kalinga, were included in the Maurya Empire under Chandragupta.