cr:7.2.2c

If (A) the active-attack or attack-source cannot move to the combat chain as a chain link, (B) the active-attack or attack-source cannot become attacking, or (C) the attack does not have any legal attack-targets[1.4.5] , instead the attack is cleared from the queue. If there is another attack in the queue, the Attack Step begins again with that attack, otherwise the Close Step begins.[7.7]

Example: Spectra is a keyword that means “This can be attacked” and “When this becomes the target of an attack, destroy this.” If a permanent with Spectra is the only target of an attack, it ceases to exist before the Attack Step and is no longer a legal attack-target, so the attack is cleared from the queue. If there are no other attacks in the queue, the combat chain closes.

Example: Kabuto of Imperial Authority has the text “opponents can’t attack with weapons.” If the attack is a weapon attack-proxy, its attack-source (the weapon) cannot become attacking, so the attack-proxy is cleared from the queue. If there are no other attacks in the queue, the combat chain closes.