Can the attacking player choose the order of their 'When I attack' triggers, and when are units considered 'in combat' for the purposes of playing Cannon Barrage as a reaction?

Ruling: When multiple triggers happen simultaneously, the controller decides the stack order, so the attacking player can sequence their attack triggers (like Anivia before Warwick). Units are considered in combat immediately when showdown starts at a contested battlefield, which occurs when moving units to an occupied battlefield resolves.

Sequence:

• Move units to contested battlefield; showdown and combat start immediately
• Attacker declares order of their attack triggers (e.g., Anivia first, Warwick last)
• Defender triggers are added to the stack
• Attacker gets priority and can play reaction-speed cards like Cannon Barrage
• Stack resolves in reverse order (Cannon Barrage, then Anivia, then Warwick)
• After all triggers resolve, attacker gets focus and priority for action-speed spells

Nuances:

• Cannon Barrage can be played as a reaction on the stack after attack triggers are declared but before they resolve, allowing it to damage enemy units before other effects like Warwick's kill trigger