"main force in" is correct and usable in written English.
You can use it when referring to the primary source of strength or energy in a particular situation or activity. For example: "The main force in the market rally was fueled by a surge in tech stocks."
Canada provides the main force in Kandahar.
Nature is the main force in shaping these 'links' on seaside land with the wind carving valleys between the dunes.
And according to Cantor, he was the main force in Washington that prevented that 2011 deal from happening.
The main force in this campaign is the Streamline Sales Tax Project, an organization representing 36 states and the District of Columbia.
No one is more infatuated than Rosemary, the beautiful young starlet who will become the main force in Dick's eventual destruction.
Its new strategy is supposed to end the practice of controlling supplies in favor of promoting demand for diamonds as the main force in the diamond business.
The parties in Stormont were close to triggering an election as the two main forces in government – Sinn Féin and the Democratic Unionist party (DUP) argued over welfare reforms.
But unless that happens it looks as if the Democrats and Thai Rak Thai will emerge as the two main forces in the new parliament.
He used his Roman veterans to execute a series of audacious flanking maneuvers while his fickle Spanish allies held the enemy's main forces in place.
In 1980 he arranged a live relay of the Berlioz Te Deum with the main forces in that venue, accompanied, through the use of loudspeakers and video cameras, by the fine organ of St Mary's Cathedral.
In 1980 he arranged a live relay of the the Berlioz Te Deum with the main forces in that venue, accompanied, through the use of loudspeakers and video cameras, by the fine organ of St Mary's Cathedral.
