Règles complètes de Magic: The Gathering

507.1.

First, if the game being played is a multiplayer game in which the active player‘s opponent‘s don‘t all automatically become defending players, the active player chooses one of his or her opponents. That player becomes the defending player. This turn-based action doesn‘t use the stack. (See rule 506.2.)

507.2.

Second, any abilities that trigger at the beginning of combat go on the stack. (See rule 603, «Handling Triggered Abilities.»)

507.3.

Third, the active player gets priority. Players may cast spells and activate abilities.