If a unit is moved from a battlefield with Flash in response to Charm, and Charm then resolves moving the unit back to that same battlefield, does this start a showdown?

Ruling: Yes, a showdown starts if the battlefield is uncontrolled after Flash resolves, because Charm is moving a unit to an uncontrolled battlefield when it resolves. You can score a point from winning the showdown if you haven't already scored that battlefield this turn.

Sequence:

• Unit is on a battlefield
• Opponent casts Charm targeting that unit
• You chain Flash, moving the unit back to base
• Flash resolves, unit moves to base (battlefield may now be uncontrolled)
• Charm resolves, opponent chooses to move unit back to the original battlefield
• If battlefield is uncontrolled, a showdown starts

Nuances:

• You can only score each battlefield once per turn, so you only get a point if you haven't already scored that battlefield this turn
• When Charm resolves, the caster chooses the destination, so even though the unit was originally on that battlefield, the changed game state makes it a legal move