"the main force of" is a perfectly valid phrase to use in written English.
You can use it to refer to the most significant or influential force behind something. For example: "The main force of the movement was the students' commitment to social justice."
He was unaware that the Archduke had arrived by forced marches from Augsburg to the vicinity of Ostrach; Jourdan thought the main force of Charles' army was still at least three days' march away.
"That is the main force, of course," Mr. Markedonov said.
And I think the youth movement will be the main force of power".
But as the morning wore on, the main force of German defenders could not be dislodged.
Unfortunately, the main force of the Mississippi was crashing against the south side of the inflow channel, producing unplanned turbulence.
As assuaging as these considerations might be, they do not blunt the main force of the objection.
The main force of the explosion was taken by his body, and he died of wounds shortly afterwards.
The main force of its actions fell on Boston, which seemed to be the centre of colonial hostility.
Since its foundation more than 20 years ago, the English Theatre Berlin has been the main force of English-language theatre throughout Germany.
Statistics are hard to come by, but the China National Tourism Administration agency says that in 2013 "the number of visitors travelling independently surpassed those who utilised organised tours to become the main force of China's outbound tourism market".
It was immediately followed by the entrance of the main force of American and Iraqi troops, repelling attacks by insurgents firing rocket-propelled grenades and answering their mortar fire with more mortar fire.
