Does moving to an already controlled battlefield open a window to respond with reactions?

Ruling: Moving in and of itself does not create a priority window. Moving to an unoccupied or enemy-controlled battlefield begins a Showdown which has priority windows, but moving to an already controlled battlefield does not.

Nuances:

• If the moving unit has a "when I move" trigger ability (like Traveling Merchant), that will create a chain offering an opportunity for players to play Reactions before Showdown begins.