J-Jarasandha

Jarasandha was the king of Magadha . He was the son of a Vedic king named Brihadratha. But he is generally held in negative light owing to his enmity with the Yadava clan in the Mahābhārata..Jarasandh decided to get his two daughters 'Asti and Prapti' married to the heir apparent of Mathura, Kansa.

When Krishna killed Kansa in Mathura, Jarasandha become enraged because of Krishna and the entire Yadavas clan on seeing his two daughters being widowed. So, Jarasandha attacked Mathura and the Yadavs 17 times. Once his army included the Panchalas and the Kurus among other kings of India. While Krishna and the Yadavs's survived the attacks, sensing danger Krishna relocated his capital city to Dwaraka. 

Dwaraka was an island and it was not possible for anyone to attack it at all. Krishna began a campaign to diminish the power of Jarasandha, one by one eliminating his powerful allies like Kalyavana, Narakshura, Hansha, Dimbaka. . In an another incident when Jarasandha arrests Sakuni and Dhuryodhana, Karna comes to the rescue. Karna fought Jarasandha to a standstill. Jarasandha gifted Malini town to Karna as a prize for his skills.

The eldest Pandava, Yudishtra wanted to perform the Rajasuya Yagya. In order to perform the Rajasuya Yagya, a king had to be declared emperor and all the surrounding kingdoms had to recognize the emperor as their overlord. In order for this to happen, Yudishtra would have to defeat Jarasandha and obtain the title of emperor

Upon reaching Magadha, Krishna, Arjuna and Bheema challenged Jarasandha to a wrestling match. Bheema and Jarasandha fought for days, both equally matched and neither succumbed to the other's blows. Bheema realizing that Jarasandha was an equal match looked to Krishna for help. Now Krishna who knew the story of Jarasandha's birth, picked up a twig from the floor, broke it in two halves and threw the two halves far away from each other.

Bheema now knew what he must do. He threw Jarasandha to the ground, held his legs and split his body in two. He then threw the two halves of Jarasandhafar away from each other so that they might not join. Bheema had defeated Jarasandha and Krishna installed Jarasandha's son as the king of Magadha. In return, Jarasandha's son agreed to be a vassal to the Pandavas

Reference:

 http://en.wikipedia.org/wiki/Jarasandha

http://www.indiaparenting.com/stories/82_2240/the-story-of-krishna-and-jarasandha.html

http://en.wikipedia.org/wiki/Magadha_Kingdom