If you control a battlefield at the start of your turn, move a unit from that battlefield to base (leaving it empty), then move another unit to that same battlefield, does open showdown happen?

Ruling: Yes, a showdown begins when you move a unit to the now-empty battlefield. However, you cannot conquer at that battlefield (for triggers like Qiuana or Candlelit), though you can regain control.

Nuances:

• The showdown that occurs is not a combat
• Rule of thumb: anytime a unit entering a battlefield could change its controller, do a showdown