Meeting d4-d6 from White's perspective How do you meet d4-d6 when your opponent doesn’t play the Old Indian, and the setup is d6-Nbd7-e5, which is Pirc territory? 

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1. d4 d6 2. null Nd7 3. null e5 


When this happens, can I play the London System, possibly with d4-Bf4, and so forth?
Planning to play the Old Indian (Be7 for black) against the London System has no bite since white can control e5 more easily. The Be7 does not aim at e5, of course, but as important, it inhibits Re8, and its control of e5 also. So minus two pieces helping play e5, and you are not likely to get that freeing move in. You would also have to play (after d6) e6 since e5 loses a pawn to any reasonable white development, Be7, and only then, figure out how to enforce e5 gaining some space in the center. In reality, you would probably have to play for c5 instead.

As a defense against the London System, the King's Indian (g6 and Bg7) is much more logical since the Bg7 can help fight for control of e5, and the Re8 immediately helps too with a potential e5. In those lines, white usually plays Bf4, but then h3, and Bh2 to anticipate black's eventual e5.