Berlin Defense, Rio Gambit Accepted

The capture 4...Nxe4 is the most direct answer to 4. O-O, as Black accepts the e-pawn that castling left unprotected. Despite the name, White's material is not seriously at risk. The pawn comes back through the open e-file while Black has yet to castle and the king remains on e8. On Lichess, 5. Re1 is clearly the most frequent reply. After 5...Nd6 6. Nxe5 a tactic stops Black from grabbing the bishop: 6...Nxb5? 7. Nxc6+ uncovers check from the rook and wins the queen. Instead 6...Nxe5 7. Rxe5+ Be7 gives the bishop time to retreat, and 6...Be7 blocks the file but allows 7. Bf1. This branch has a reputation as highly drawish.

Theory treats 5. d4 as the main line. Here 5...exd4? is a mistake because 6. Re1 pins and wins the knight, so Black replies 5...Nd6, the route that usually ends in the Berlin endgame. Results after 5. d4 are almost evenly split, whereas the rarer 5. Bxc6 has scored well for White. The engine judges the position roughly balanced.