driving force
The primary instigator, catalyst, or motivator that compels or directs something to happen. Major financial losses were the driving force behind the layoffs. For over 40 years she has been the driving force of this company, so her retirement leaves its future somewhat uncertain. He dreamed of being a driving force in his country's government someday.
*the driving force (behind someone or something )
the person or a thing that motivates or directs someone or something. (*Typically: be ~; become ~; serve as~.) Making money is the driving force behind most businesses. Ambition is the driving force behind Tom. Love can also be the driving force.
the driving ˈforce (behind something)
the person or thing that makes something happen: She is the driving force behind this new road safety campaign.