411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nf6/3. Nxe5/3...d6/4.Nd3
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4. Nd3?! — Petrov's Defense: Karklins-Martinovsky Variation
4. Nd3?! retreats the knight to d3, keeping the piece safe while avoiding the more common retreats to f3 or c4.
Downsides: The main drawback of 4.Nd3?! is that the knight on d3 blocks the d-pawn and may interfere with White's central pawn development.
Theory Table:
1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6 4. Nd3