Question: why bhishma and parshuram fight?
Answer: Parshuram and Bhishma fought with each other as Bhishma denied marriage proposal of Parshuram - to marry Amba – a princess of Kashi. Why Parshuran wanted Bhishma to marry Amba? Bhishma abducted Amba as a bride for his brother- Vichitravirya. But, Amba was in love with Salwa – the ruler of Saubala.