This variation is the most aggressive White could play. By playing c4, White threatens either to build a very powerful center or to destroy Black's army at the queen side.

Black can react in various ways.

3...Bb7 ignores the threat and strikes at the center. It wouldn't be wise for White to trade a wing pawn against a center pawn. But now, White can defend with Nc3 as b4 would be answered with Qb3 (pinning the pawn).

3...b4 tries to keep the pressure at the queen side, letting White build his center. It's a very sharp variation for Black as White is going to have many weapons to attack both at the center and at the queen side, including moves as Qb3, d5, Nc3 and Bd2.

3...bxc4 isn't very logical, as Black abandons his queen-side power. He will have to fight back at the center despite White advance in development. In this case, it would have been better to play a classical Open Game (1...e5) or Sicilian (1...c5).

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'1.e4 a6 2.d4 b5 3.c4'

Wikipedia has related information at St. George Defence.