The ruling Nanda King insulted another man named Chanikya, and Chanikya told one of his students, Chandragupta Maurya, to curse the Nanda King, Dhana Nanda, that his Kingdom would be lost.
Years later, around 321 BCE, Chandragupta Maurya took control of Bangladesh and most of Northern India, the Nanda Empire.
He then conquered most of the surrounding countries, and by 273 BCE he had a huge empire. He even defeated the powerful Seleucid empire (One of the successors to Alexander's giant kingdom who took much of Alexander's empire by fighting against Antigonus), winning territory west of the Indus river. Seleucus could have been killed by the Mauryans but they let him go, for much territory in Iran, Uzbekistan, etc.
Chandragupta retired by 297 BCE, and his son Bindusara got the throne. Bindusara took over nearly all of Southern India, with the exception of Kalinga (Odisha) on the Eastern Coast, and the southernmost part of Deccan. He attempted to take South Deccan but failed.
In 268 BCE, after a minor struggle for the throne, Chandragupta's grandson and Bindusara's son, Ashoka, inherited the throne. He fought a bitter battle with the stubbornly audacious Kalingas. The Mauryan army of 639,000 was huge. Macedonia, the empire of Alexander the Great, had been scared away by around 298,000. To get a sense of how big that really is, take this into consideration: The extremely powerful empire of Alexander III had only 40,000! This army of 639,000 was so big that it could have taken over the whole world in a flash. Nearly all countries now have smaller armies, despite drastic population increases and much better recruition systems. The gigantic Kalinga army of 161,000 fought hard; Nevertheless, it was completely crushed. However, Ashoka said that there were too many deaths, and he became a good ruler who didn't fight to expand his empire. Because of this, he has earned the title Ashoka the Great.
Ashoka also managed to take over the entire Deccan (South India.)
However, Ashoka was not some sort of philosopher, though he visited his people daily, renounced fighting, and became a Buddhist. He used his army well; any strong invasion was treated like it might as well have not existed by Ashoka's army, same thing with rebellions.
After Ashoka's death in 232 BCE, the Maurya Empire became weaker, losing land to the Satavahanas in the south, and in 185 BCE, following the assassination of King Brihadratha Maurya, by his general Pushyamitra Shunga, the empire collapsed, and was replaced by the rather weak Shunga empire.
