A what if question.

[ohtatyaro]

It would not have happened, I mean, look at the Tale of Years:

2948
Théoden son of Thengel, King of Rohan, born.

2951
Sauron declares himself openly and gathers power in Mordor. He begins the rebuilding of Barad-dûr.

In times of Fengel, there was not 'full strengh' of Sauron yet. I mean, the massive attack was not possible, as he only begins to gather it when already Thengel is the King, as Fengel died in 2953.