Why was Nd2 played? instead of f4! 1kb4r/4r3/p1p3p1/Pp1p1p1p/1P1P3P/5PP1/2P2K2/1NR2R2 w - - 0 1

1. Nd2 Rhe8 (1... f4!) (1. f4!)


Silman suggests 1. Nd2, with a black response of Rhe8. Wouldn't black just respond with 1...f4! instead? This gives black counter-play on the kingside, opens an f-file for him, and activates his bishop (with an eventual ...Bf5!)

White should have just played 1. f4! What prevented such a move from being considered?
Why 1. Nd2?

Silman is suggesting to improve the worst placed piece with Nd2-b3-c5. This makes sense because:


White will have a good knight vs. bad bishop
The position is closed so White has the time to improve the worst piece


Why not 1. f4?

Black's ...f4 and ...Bf5 plan isn't that strong, because:


When black's bishop gets to f5, it doesn't threaten anything
White doesn't have to open files with gxf4. White should continue with the Nd2-b3-c5 plan as before


Having said this, 1. f4 isn't a bad move, i.e. it deprives the bishop of another target.