Can Consult the Past be activated as a reaction when an opponent plays a unit to their base or uses a spell not targeted at the battlefield with Consult the Past, or must you wait until they directly attack that battlefield?

Ruling: You can activate Consult the Past as a reaction to any trigger, spell, or anything that starts a chain. You don't have to wait for your opponent to do something on a specific battlefield.

Sequence:

• When a unit is played, it goes on the chain but resolves immediately with no window to respond
• If the unit has an effect after entering (like an ETB ability), that effect activates and starts its own chain, which you can respond to
• Spells linger on the chain, allowing reactions
• Permanents (units, gears) leave the chain as soon as they enter and resolve immediately

Nuances:

• You cannot react to units or gear being played unless they have triggered effects after entering
• Permanents only immediately resolve when they are the first item on the chain; if a permanent is on the chain but not as the first item, players can play reactions on top of it
• This immediate resolution rule is a technical aspect where units start a chain but resolve with no window for reaction