Historical Rules · 20 September 2024
Rule 809.3c
Official source text from the 20 September 2024 Rules release
A player can attack only an opponent seated immediately next to them, a planeswalker controlled by a player seated immediately next to them, or a battle protected by a player seated immediately next to them. Example: At the start of an Emperor game, neither emperor can attack any opponents, even though both of the opposing generals are within their spell range.