Here's the position after 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O b5 6.Bb3 Bc5
[StartPly "12"] [FEN ""] 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O b5 6.Bb3 Bc5
There are 2 main continuations for White: 7.c3 (the most obvious imho) and 7.a4. Actually there's also the modestly looking 7.d3 which is good, no doubt, but I'm not interested in the d3 system at the moment)
What are the pros and cons of the the both moves/lines?
And what does White really achieve (or try to achieve) by attacking the pawn on b5? I understand it cannot be recaptured by the a-pawn, so Black would normally defend it with his rook or go Bb7. Second, I'd also like to understand why 7...b4 is bad from positional point of view.
Also, sometimes White precedes a4 with c3. So, instead of playing a4 right away he waits for 7...d6 and only then plays 8.a4 (again instead of the obvious to me 8.d4). What is the idea here?
[StartPly "15"] [FEN ""] 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O b5 6.Bb3 Bc5 7.c3 d6 8.a4