Does Imperial Decree affect units that enter the battlefield after it has been played?

Ruling: Imperial Decree affects all units, even if they entered after it was played. The trigger goes on the chain during the combat damage step and players can react to it.

Sequence:

• Imperial Decree creates a lingering effect that triggers when a unit takes damage
• During the Combat Damage Step, when units take damage, Imperial Decree's trigger goes on the chain
• Players can respond to the trigger while it's on the chain
• However, you cannot save a unit that already took damage in combat, because the trigger to kill it is already on the chain

Nuances:

• Imperial Decree does not modify units themselves; it creates a lingering effect that applies to any unit that takes damage after it resolves
• This is different from effects like Grand Strategem which only affect units present when it resolves
• There is no official rule text clarifying which effects create lingering effects versus one-time modifications