From the scriptures, we know that Jarasandha attacked Mathura 17 times and was defeated each time. Finally, on the 18th attack (along with Kalayavana), Lord Krishna moved the Yadavas to the island city of Dwarka.
My question is what is the significance of the 17 attacks ? Why did either side wait for the 17 attacks ? Why did Lord Krishna wait for 17 attacks to decide to move the Yadavas ? Why not move it after the third or fourth or tenth or twelfth attack ? Why did Jarasandha attack 17 times before joining forces with Kalayavana ? Why not do so in the first attack (and consequently having a much bigger force) ?