Black is going to play ...e6 to generate counterplay in the center. This pawn break is harder to achieve when black has a knight on d7, which blocks both the d8-queen and c8-bishop. Therefore white provokes ...Nb8-d7 with 6. Bb5+, and he’ll try to prove that delaying ...e6 costs black more than what his extra tempo (Bf1-b5-e2 wastes a tempo compared to 6. Be2) can give.
6. Bb5 Nbd7 is not the only option for black, though. Both 6... Bd7 (if white returns his LSB to e2, black will play ...Bg4 and tempo is equal) and 6... Nfd7 (increases control of e5, and the b8-knight will develop to c7 via a6) are playable.
You are White in this position? What do you do?
(I'll give a moment to think about it)...
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Well the most popular move (barely) is Be2. However both the chess engine and the win percentage from the openings database say that Bb5+ is better. Now black can block with either knight to d7 or the Bishop to D7. Your best move is A4 (to prevent Qa4+), but then black is likely to play a6 (threatening the bishop), to which the bishop retreats to e2.
So if you had moved straight to e2 you'd end up here:
And this way you end up with this board:
What am I missing? Is there some advantage for white of a5? Thanks for the help... I often look at openings and think "Why is that???" and this is one of those times.