How does Whirlwind work? Can players choose any unit to return to hand, including opponents' units?

Ruling: Each player chooses any unit on the battlefield to return to hand, not just their own units. The "may" is included so players aren't forced to return their own units if there aren't enough opponent units available.

Nuances:

• Players can choose opponent units to return
• Opponents can also choose any unit, including yours
• The "may" prevents forcing you to bounce your own units when insufficient targets exist