Rule 508.4

If a creature is put onto the battlefield attacking, its controller chooses which defending player, planeswalker a defending player controls, or battle a defending player protects it’s attacking as it enters the battlefield (unless the effect that put it onto the battlefield specifies what it’s attacking). Similarly, if an effect states that a creature is attacking, its controller chooses which defending player, planeswalker a defending player controls, or battle a defending player protects it’s attacking (unless the effect has already specified). Such creatures are “attacking” but, for the purposes of trigger events and effects, they never “attacked.” They remain attacking creatures until they’re removed from combat or the combat phase ends, whichever comes first.

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