Does playing a unit on a battlefield you already control with another unit trigger a showdown?

Ruling: No, playing a unit on a battlefield you already control does not trigger a showdown. Moving a unit to a battlefield you control also does not trigger a showdown.

Nuances:

• Showdowns only happen when a unit applies contested status, which requires an enemy-controlled or uncontrolled battlefield
• If you move your only defending unit away from a battlefield (making it uncontrolled) and then move another unit back to that battlefield, it will trigger a showdown
• If you move a second unit into a battlefield first, then move the first unit out, no showdown is triggered