Can you use Cannon Barrage to kill a unit in response to Fortified Positions' battlefield trigger before the unit gains shield?

Ruling: Yes, you can respond to the Fortified Positions battlefield trigger with Cannon Barrage to kill the unit before it gains shield. The battlefield trigger that grants shield goes on the chain when combat begins, and you can play a reaction when you have priority.

Sequence:

• Move occurs
• Passive abilities turn on
• Combat triggers (including Fortified Positions) go on the chain
• Players can respond with reactions like Cannon Barrage

Nuances:

• Shield itself is a passive ability, but the trigger that grants shield is a triggered ability you can respond to
• If a unit already has shield as a baseline ability before combat (not from a trigger), you cannot respond to kill it since there's no trigger to respond to
• The showdown starting happens in the cleanup after the move, unless there is a move trigger