Chapter 9 of the PHB / ground rules describes how combat actions work.
The sidebar of "Combat step by step" in the "Combat Order" section lists how the combat proceeds:
- Determine the surprise. The DM determines if someone involved in the combat is surprised.
- Establish positions. The DM decides where all the characters and monsters are. Given the marching order of the adventurers or their declared positions in the room or elsewhere, the DM realizes where the adversaries are, at what distance and in what direction.
- Roll initiative All those involved in combat combat launch the initiative, determining the order of the combatants' turns.
- Take turns. Each participant in the battle takes a turn in order of initiative.
- The next round begins When everyone involved in the fight has had a turn, the round ends. Repeat step 4 until the fight stops.
As you can see, it does not indicate anywhere in this macro view that it declares its actions before taking its turn.
Looking at the "Your Turn" section, we can see the types of things you can do on your turn:
On your turn, you can movement a distance up to your speed and take an action. You decide whether to move first or take your action first. Your speed, sometimes called your walking speed, is recorded on your character's blade.
The most common actions you can perform are described in the Actions in combat section. Many class features and other skills provide additional options for your action.
The Movement and Position section gives the rules for your movement.
You can give up moving, perform an action or do anything in your turn. If you can not decide what to do on your turn, consider performing the Dodge or Ready action, as described in "Actions in combat."
The section continues describing the bonus actions:
Various class features, spells and other abilities allow you to perform additional action on your turn called bonus action. […] You can perform a bonus action only when a special ability, spell or other game feature indicates that you can do something as a bonus action. Otherwise, you do not have a bonus action to take.
And another activity on your turn:
You can communicate as you can, through brief expressions and gestures, as your turn comes.
You can also interact with an object or characteristic of the environment for free, either during your movement or your action. For example, you can open a door during your movement while advancing towards an enemy, or you can draw your weapon as part of the same action you use to attack.
If you want to interact with a second object, you need to use its action. Some magical objects and other special objects always require an action to use, as indicated in their descriptions.
And even reactions:
Certain special abilities, spells and situations allow you to perform a special action called reaction. A reaction is an instantaneous response to a trigger of some kind, which may occur in your turn or in that of another person. The opportunity attack, described later in this section, is the most common type of reaction.
… But as you can see, none of these rules states that you must declare the action you intend to take at any time; it only says that you take them (some at any time, others when a certain condition is met or when a characteristic / spell allows you to take them).
Later, in the "Movement and position" section, it tells you how the movement works:
On your turn, you can move a distance up to your speed. You can use as much or as little of your speed as you wish on your turn, following the rules here.
He can break his movement on his turn, using some of his speed before and after his action. For example, if you have a speed of 30 feet, you can move 10 feet, act and then move 20 feet.
You can even move between the attacks:
If you perform an action that includes more than one weapon attack, you can divide your movement even more by moving between those attacks. For example, a fighter who can perform two attacks with the Extra Attack function and who has a speed of 25 feet could move 10 feet, make an attack, move 15 feet and then attack again.
Once again, there is no mention of declaring your actions.
As you can see, there is nothing in the rules that requires you to declare your actions in advance, simply do the things you want to do in your turn, if you have the motion, the action or the bonus action (or any other resource you take) . ) available to do so.
The D & D combat is sequential, without an action declaration phase at the beginning. Your turn can also be interrupted by someone's reaction. Such an interruption could, among other things, incapacitate him, which means that his intention to take some action was never fulfilled.
There are, in fact, tons of flexibility in how certain things can be ordered in combat. But if one thing is conditioned to another, they must happen in order, since the intention has no weight in the rules of engagement, since it could be interrupted at any time and incapacitated.
D & D combat does not have an action declaration phase. Things happen in order, and can be interrupted at any time by a reaction, a trap or something similar. You can say: "I plan to take the Attack action", but that has no relevance to the rules until you are taking the action.
In D & D, the way you take action in combat is actually taking the action. There is no declaration phase of actions. Flurry of Blows occurs after the attack action, which means the action itself, not a statement that you will take the action.