Chess Opening Theory/1. e4/1...a6/2. d4/2...b5/3. c4/3...Bb7/4. Nc3/4...b4

= St. George Defence/Three Pawns Attack = Most knight moves would lose a pawn - but /5. Nd5/ prevents this and grabs at the centre. White can also play /5. Qb3/ - which is the recommended move.

Theory table
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1.e4 a6 2.d4 b5 3.c4 Bb7 4. Nc3 b4