When my opponent moves to an open battlefield starting a showdown, passes focus, and I play a unit with Ambush and pass, does my opponent get focus again or does combat start immediately?

Ruling: Combat starts immediately. After your unit resolves and enters the battlefield, a cleanup occurs. Because opposing units are now present at the contested battlefield, combat becomes staged. Per the updated rules, the non-combat showdown immediately transitions into a Combat Showdown without requiring both players to pass consecutively.

Sequence:

• Opponent moves to open battlefield, showdown begins
• Opponent passes focus
• You play unit with Ambush, unit resolves and enters battlefield
• Cleanup occurs, combat becomes staged
• Showdown immediately becomes Combat Showdown
• Attacker and Defender are designated
• Initial chain is created with "When I Attack" and "When I Defend" triggers
• After initial chain resolves, Attacker gains focus

Nuances:

• The rule change eliminated the requirement to wait for consecutive passes before transitioning to combat
• This applies when opposing units become present at a battlefield where a showdown is ongoing