Magic: The Gathering Comprehensive Rules

507. Beginning of Combat Step

• 507.1. First, if the game being played is a multiplayer game in which the active player’s opponents don’t all automatically become defending players, the active player chooses one of their opponents. That player becomes the defending player. This turn-based action doesn’t use the stack. (See rule 506.2.)
• 507.2. Second, the active player gets priority. (See rule 117, “Timing and Priority.”)