411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...c6/2. d4/2...d5/3. f3/3...e6/4. Nc3/4...Bb4/5. Ne2/5...Ne7/6. a3
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Caro-Kann Fantasy Variation B12 Caro-Kann → 2...d5
6. a3 · Caro-Kann Fantasy Variation
Black certainly doesn't want to exchange the strong black bishop for the knight on c3, and free up e2 to allow White's light-squared bishop to be developed.
Black could continue after 6...Ba5 by developing the knight to d7, castling, and then attempting to break in the centre.
Theory table
6... Ba5 6... Bd6
References
See also
Caro-Kann