Can I use Cleave to boost my attacking unit after my opponent uses Smoke Screen in response to my battlefield move?

Ruling: You cannot use Cleave as a reaction to Smoke Screen because Cleave is an action, not a reaction. However, after Smoke Screen resolves, you regain focus and can then play Cleave to boost your unit.

Sequence:

• Attacker moves to battlefield and passes priority
• Defender uses Smoke Screen (reaction)
• Smoke Screen resolves and its effect snapshots in place
• Attacker regains focus and can now play Cleave (action)
• Whether Cleave is sufficient to overcome the Smoke Screen debuff depends on the specific units involved

Nuances:

• Cleave is an action and cannot be used to react to anything
• Smoke Screen's effect snapshots when it resolves, so subsequent stat changes from Cleave are calculated separately