natural tendency; natural course of events; force of circumstances
Refers to the way things naturally unfold without external intervention; often used when events proceed according to their own momentum or inherent logic.
By the natural course of events, he became the leader.
It's best to let things take their natural course.
自然 alone can mean 'nature' or 'natural', but 自然の勢い specifically refers to the force or momentum of natural development.
A compound of 自然 (nature, natural) and 勢い (force, momentum). The phrase is transparent and follows standard Japanese compounding patterns.