Enchantment, 3W (4) At the beginning of each opponent's combat phase, separate all creatures that player controls into two piles. Only creatures in the pile of his or her choice can attack this turn.1
Illus. Randy Gallegos
Creatures which comes into play (or cards which become creatures) after the beginning of combat ability resolves are not placed in either pile and are not affected by this ability. [Jordan 2001/02/13]
The effect is best understood if you read it as "creatures in the pile the player does not choose cannot attack this turn." [D'Angelo 2002/06/18]
If you have two of these, then two of these effects will be applied in order. The first time, the player gets to pick a set that can attack and the other set cannot. The second time, it does not matter what you do with the set that could not attack, so you split up the ones that still could attack. [D'Angelo 2002/06/18]
Invasion
(Rare)