War Falcon

• WWizards of the Coast2012-07-01
  Official Ruling
  If an effect makes War Falcon a Knight or a Soldier, it will be able to attack even if it's the only creature you control.
• WWizards of the Coast2012-07-01
  Official Ruling
  To attack with War Falcon, you must control a Knight or a Soldier when you declare attackers. Once War Falcon is attacking, it won't be removed from combat if at any point you no longer control a Knight or Soldier.

Rulings via Scryfall