DMPQ- Why did Magadha emerged as the most formidable mahajanapada?

Between the 6^th and 4^th century BCE, Magadha became the most important mahajanapada. The possible reasons could be:

1. It was a region which was highly productive and hence surplus production was suitable for maintaining huge army.
2. Magadha had huge resource of Iron. This metal was used both for weapons and tools. Latter increased the productivity of the land and former increased the offensive power.
3. Elephants an important part of the warfare were found in abundance.
4. Moreover geographical location make it impregnable as fort. As it was surrounded by rivers. These rivers also played a role in movement of troops.
5. Early Buddhist and Jaina writers who wrote about Magadha attributed its power to the policies of a number of ruthlessly ambitious kings of whom Bimbisara, Ajatashatru and Mahapadma Nanda are the best known.