Chess Opening Theory/1. e4/1...c5/2. Nf3/2...d6/3. d4/3...cxd4/4. Nxd4/4...Nf6

= Open Sicilian = White's pawn is under attack from the knight on f6. They must defend it or forestall Black's capture of it in some way. /5. Nc3/ is almost universally played as it is the knight's natural square anyway. A knight on c3 increases White's influence of the d5 square, and forms a defensive bulwark against attacks on the c-file, to the extent that Black will often give up a rook to remove it.

Also played is /5. f3/. At first glance White is attempting to reserve the option of moving the c-pawn to c4. However there is not usually time for this, because without a White knight on c3 Black is immediately threatening to play e5 and d5 and equalise in the centre. White is compelled to throw a spanner in the works, usually with Bb5+, and c4 gets forgotten in the ensuing complications.

/5. Bb5/+ is plausible and may transpose into common lines, but if White's plan is to simplify this way, why now and not two moves previously when White could have followed up by castling kingside with a useful defensive knight on f3? Bb5+ is a shade inconsistent.

/5. Bd3/ commits the bishop to a square on which it will have no future after the obvious e5. It also fails to prevent a subsequent d5.

/5. Bc4/ is playable, apparently dropping the e4 pawn, but intending Qh5; after which, if Black plays e6 to block the attack on f7, White has ideas of Bb5+ and Nxe6.

Theory table
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1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6