How to resolve an endless loop of attacks of opportunity?

Question

Combatant Alice and combatant Bob, both wielding a dagger in one hand, strike each other using an off-hand unarmed attack. No one has the Improved Unarmed Strike feat.

Assuming that you can make AoO against other AoO, and you can make AoO using unarmed strike, their actions will be:

1. Alice tries to hit Bob, triggering the AoO from Bob
2. Bob tries to hit Alice with his AoO, using unarmed strike, triggers AoO as well
3. Alice tries to hit Bob with her AoO, using unarmed strike, triggers AoO
4. (go to 2)

How this endless loop of attempts should be resolved? Who attacks first?

Answer 1

It's not an endless loop

Combat on Attack on Unarmed Strike says, "An unarmed character can’t take attacks of opportunity (but see 'Armed' Unarmed Attacks, below)." "Armed" Unarmed Attacks says

Sometimes a character’s or creature’s unarmed attack counts as an armed attack. A monk, a character with the Improved Unarmed Strike feat, a spellcaster delivering a touch attack spell, and a creature with natural physical weapons all count as being armed….

Thus when Alice—who lacks the feat Improved Unarmed Strike—makes an attack with her unarmed strike against Bob, Alice provokes an attack of opportunity from Bob. But Bob—who also lacks the feat Improved Unarmed Strike—can't make attacks of opportunity with his own normal unarmed strike. Were Bob to possess the feat Improved Unarmed Strike—or employ his dagger—, he'd make his attack of opportunity normally with that appropriate weapon due to Alice's attack and wouldn't himself provoke.

Despite both wielding daggers, wielding a weapon doesn't obviate the attack of opportunity provoked by attacking with a normal unarmed strike nor does wielding a dagger allow a combatant to threaten with the combatant's normal unarmed strike. As per Attacks of Opportunity on Threatened Squares: "If you’re unarmed, you don’t normally threaten any squares and thus can’t make attacks of opportunity." When making a normal unarmed strike, both Bob and Alice are unarmed.

(Note that also both Bob and Alice would both have to possess the feat Combat Reflexes or something similar to even make multiple attacks of opportunity in a round, usually eliminating the possibility of this situation becoming an endless loop that way, too.)

Answer 2

This is resolved with a maximum of AoO per (turn|round).

Taking Pathfinder rules for example. A character is limited to (dex modifier) attack of opportunity per turn is it has Combat reflexes feat. Otherwise, it is limited to one

Answer 3

The loop wouldn't be endless, as they only have a limited number of AoOs (I'm assuming, I don't know if you can get infinite AoOs?)

They can only declare as many as they have, and once they've expended their allotment, they can no longer 'queue' up any more.

The key here is that opportunity attacks 'interrupt' actions and occur before the triggering action is resolved. But the triggering action is not cancelled. Think of it as chain of tokens, where each player places their attacks before the other until they're both out of AoOs.

Answer 4

Whoever got the last attack of opportunity attacks first

From the combat rules, we know that attacks of opportunity interrupt the flow of actions:

An attack of opportunity “interrupts” the normal flow of actions in the round. If an attack of opportunity is provoked, immediately resolve the attack of opportunity, then continue with the next character’s turn (or complete the current turn, if the attack of opportunity was provoked in the midst of a character’s turn).

And are resolved first based on what action provoked the attack of opportunity, as shown on this post from Jason Bulmahn (by then, Lead Designer at Paizo):

I am kinda amazed that this is still raging on. I have skimmed the posts from my ruling till now and most of them seem to be focused around a gamist argument, which I can understand. The time issue really is just to keep matters simple (as many have pointed out). Technically, the AoO occurs as the event that provokes it is taking place, but since we can't have "middle ground" conditions, they are pushed to before to keep things straightforward. This is the only way it makes sense for spellcasting, movement, and, in this case, standing up and trip.

So, if an action provokes an attack of opportunity, the attack of opportunity is resolved before the action that provoked it. If multiple actions provoke multiple attacks of opportunity, then you will probably have to write down which provoked and which are attacks of opportunity for ease of controlling the situation. That might not be necessary when the combatants can only make one or two attacks of opportunity per round, unless they both have high dexterity and combat reflexes.

Example: Two characters are fighting, character A attacks character B with a ranged weapon at melee range and provokes an attack of opportunity, character B then attempts to disarm character A and also provokes another attack of opportunity because character A has improved unarmed strike and can threaten with his feet. This is the apparent order of events:

• 1: Ranged attack;
• 2: Disarm attempt;
• 3: Kick to the face.

But this is the order things are actually resolved in the game system:

• 1: Kick to the face;
• 2: Disarm attempt;
• 3: Ranged attack.

Should the kick to the face put character B unconscious, his disarm attempt automatically fails because you cannot act while unconscious. Should the disarm attempt be successful, the ranged attack fails because the character now has no weapon, though he could draw another weapon using his move action now.

Another example of this is the trip FAQ, which states that a character who is standing from prone and provokes an attack of opportunity, cannot be tripped again because he is technically still prone when the attack happens.