Imperial Decree

Per rule 443.1.d.3, you must assign lethal damage to a unit in combat before assigning damage to another unit.

443.1.d.3. Units must have lethal damage assigned to them in full before damage is assigned to a different Unit. Example: If a player has 5 damage to distribute among four 3 Might units, they may not choose to assign 2 damage to one of the units and 1 damage to each of the remaining 3. They must assign at least 3 damage to one, and the remaining 2 to another.

Imperial Decree do no make damage lethal as assignment step, thus, you cannot assign 1 damage to multiple ennemy units in order to kill them. You must assign damage as normal as if Imperial Decree wasn't there. Then, Imperial Decree would kill all units that were dealt damage.