We need to find the root of the problem.
争いの根源は資源の不足にある。
The source of the conflict lies in the scarcity of resources.
He tried to get to the root of the problem.
Poverty lies at the root of crime.
That misunderstanding became the root cause of all the conflicts.
根源 is a formal, abstract word for the ultimate source or cause of something, while 根 is the everyday word that covers both literal plant roots and figurative causes.