Can a creature react to being targeted by a missile attack from a non-Hidden foe?

Question

Making an attack has the following structure:

1. Choose a target.
2. Determine modifiers.
3. Resolve the attack (typically as a hit or miss)

Consider a creature, standing near an ally, who could potentially be attacked by missile fire from a visible opponent.

On the creature's turn, can it Ready an action such as "If I am targeted by the archer, I drop prone," so that it can have the prone condition during Step 2 of the attack, imposing Disadvantage on the attack roll?

Or, rather, must it say "If I am attacked by the archer, I drop prone," such that it being prone resolves after the attack has been made and thus does not affect the attack itself?

Answer 1

Just take the Dodge action.

Taking the Dodge action achieves the desired result, and more:

When you take the Dodge action, you focus entirely on avoiding attacks. Until the start of your next turn, any attack roll made against you has disadvantage if you can see the attacker, and you make Dexterity saving throws with advantage.

This is way better than using your action to Ready dropping Prone:

• A prone creature's only movement option is to crawl, unless it stands up and thereby ends the condition.
• The creature has disadvantage on attack rolls.
• An attack roll against the creature has advantage if the attacker is within 5 feet of the creature. Otherwise, the attack roll has disadvantage.

This has the further advantage of not having to rely on your DM to approve the wording of the trigger for your readied action or its timing.

Answer 2

Things can happen when something is targeted by something and before anything else happens after the targeting

Examples of such are the Goblin Boss, which has the "Redirect Attack" reaction:

When a creature the goblin can see targets it with an attack, the goblin chooses another goblin within 5 feet of it. The two goblins swap places, and the chosen goblin becomes the target instead.

And the Mastermind Rogue's Misdirection feature:

When you are targeted by an attack while a creature within 5 feet of you is granting you cover against that attack, you can use your reaction to have the attack target that creature instead of you.

There are a few other features that similar redirect attacks or that occur when you are targeted but before you actually take damage or suffer the effects of whatever is targeting you. From these sorts of features we know things can happen after you are targeted but before anything else happens.

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Whether you can have a Readied action occur then is up to the GM

The Ready action lets you use whatever perceivable trigger you want and the action you've Readied occurs immediately after that trigger. Unfortunately, with the reaction examples listed above it is unclear whether they are happening after their trigger or even when "after being targeted" is. This is going to be left up to the GM, but we do have some similar questions about trying to do things after being targeted but before being hit, which may provide some insight on previous discussion and GM rulings:

• If I ready an action to an enemy's attack targeting my ally, and my reaction helps my ally move, does the enemy get to retarget?
• When the artificer's Mechanical Servant uses its reaction to make a melee attack, does it interrupt the triggering attack against the artificer?
• Can a thrown javelin be catapult-ed back?
• Can you cast the Darkness spell on an in-flight arrow using the Ready action?
• Can you use a readied action to teleport out of the way of a weapon attack?
• Is it possible to interrupt spellcasting?

Answer 3

Don't use words like "targeted"; use words about perceptions

The trigger for readied actions must be perceivable. In other words, seen, heard, felt, other.

So, what is it that the character is going to see, hear, feel or otherwise perceive?

"I ready an action for when the archer points an arrow at me."

Characters in the game world don't have any conception of targeting, attack rolls, saving throws, and other game terms. They only see, hear and feel things in their world.

Answer 4

Can the targeting itself be perceived?

The rules for Readying an action (PHB 193) say (emphasis mine):

Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn. First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger...When the trigger occurs, you [can] take your reaction right after the trigger finishes

If being targeted in and of itself is a "perceivable circumstance", then a creature with a Reaction available could respond to the targeting itself, and have its response occur "right after the trigger finishes" and before Steps 2 and 3, potentially influencing what modifiers are applied and thus whether or not it is hit. If, on the other hand, simply being targeted is not "perceivable", then the target would need to wait until the attack itself was resolved in Step 3, and any potential response would thus come after it was either hit or missed, too late to influence the result.

In terms of a rules basis for allowing a creature to perceive that it is being targeted, it is clear that several game features (see below) permit just that. Absent these abilities, however, it is a DM's call as to whether the targeting itself can be perceived (and thus, reacted to) under the general principle of "The DM describes the environment". In the OP's case, the DM might permit a reaction when the creature in question realizes that the archer is drawing on them, but might say that the creature cannot distinguish between the archer aiming at them or their ally and so only allow a reaction whose trigger was either of them being targeted, but not the creature specifically. Or the DM might say that the archer's attack is simply 'too fast' for the creature to react to the targeting before the attack itself resolved.

Analyzing existing abilities reveals a pattern

Several game features (see below) already give creatures the ability to both detect that they (or others) are the targets of an attack, and then to respond to that targeting before the resolution of the attack itself. It could be that the initial ability to detect and respond to the targeting is available to any creature with a carefully-worded readied action, and that the game features only add more options for the actual responses themselves. Or, it could be that the game features themselves add the ability to detect and respond to targeting, and that any creatures without these features are restricted to responding to completed attacks instead.

Unfortunately, the current rules do not distinguish between these possibilities. As such, it is your call as a DM whether you want to increase the complexity of readied actions and reactions by allowing them to trigger off of the targeting step of attacks. If you do not want to allow this, the simplest thing to rule is that absent a specific ability that allows them to react to targeting as a trigger, no creatures have this ability, and even those who do can use it only as directed by the ability itself.

However, if you do wish to allow creatures to respond to being targeted, it is worthwhile to do a pattern analysis on the already existing abilities:

A creature with the Protection Fighting Style (PHB 72) may respond as a Reaction to an attack on another target before the attack hits:

When a creature you can see attacks a target other than you that is within 5 feet of you, you can use your reaction to impose disadvantage on the attack roll. You must be wielding a shield.

A goblin boss (MM 166) may use Redirect Attack as a Reaction when it is targeted, provided it can see the attacker:

When a creature the goblin can see targets it with an attack, the goblin chooses another goblin within 5 feet of it. The two goblins swap places, and the chosen goblin becomes the target instead.

A Mastermind Rogue can use their Misdirection ability (XGtE 46, and thanks to Exempt-Medic for the find) as a Reaction when they are targeted:

When you are targeted by an attack while a creature within 5 feet of you is granting you cover against that attack, you can use your reaction to have the attack target that creature instead of you.

A creature with the Mounted Combatant Feat (PHB 168) may respond to its mount being targeted, and does not even have to use its Reaction or see the attacker to do so:

You can force an attack targeted at your mount to target you instead.

Considering all of these features, what becomes clear is that none of them cancel the attack itself or leave it without a target.¹ The Protection Fighting Style keeps the same target and simply applies Disadvantage during Step 2. All of the others change the target, but allow the attack itself to proceed through Steps 2 and 3 to its natural resolution.

In contrast, allowing any creature to take otherwise acceptable reactions that would interrupt an attack could easily disrupt the resolution of that attack. For example, in the OP's case, allowing a creature to drop prone in response to being targeted by an attack is fine - until it drops prone behind an obstacle that gives it total cover. Since total cover should prevent it from being targeted at all, it is unclear how to proceed with Steps 2 and 3 of the attack. Similarly, if the GM allows a PC to ready a spell for casting in the event that it is targeted, what happens if the PC selects a spell such as dimension door and then moves themselves behind cover or out of range?

If you are going to permit creatures to react to being targeted, you may wish to follow the model of the already existing abilities and require that reactions do not invalidate the attack itself, but merely apply modifiers or switch the target.

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¹_{Exempt-Medic Points out that the Mounted Combatant Feat, because it allows the user to switch the target to themselves, can permit someone normally untargetable (because of issues of reach or cover) to become a target and thus contradict my inferred principle that such abilities cannot leave an attack without a target. For full discussion see this question and this question, but my inclination at this point would be to rule that by using the Mounted Combatant feat, a rider is deliberately choosing to expose themselves in order to protect their mount, and is thus voluntarily coming within reach or breaking cover.}