Take a commoner who’s more physically fit than average, put a spear in his or her hand, give him or her some combat training, and you have a guard, the first line of defense against player characters who might otherwise run amok through the picturesque towns of your campaign setting.
With their above-average (though not exceptionally so) physical ability scores, guards are well suited for the simple, direct combat role of “Go ye forth and poke it, then poke it again.” That being said, they are essentially nothing more than well-trained commoners. Braver than average they are, and more motivated by duty, but this presumes that 99 times out of 100 they’re not facing any foe more challenging than another commoner. Throw a monster at them, and they’re as likely to flee or freeze as they are to stand and fight. They know how to use their weapons, but that’s not the same as understanding strategy and tactics; their sophistication extends only as far as knowing that a surrounded foe is less likely to get away, so if they outnumber their opponents, they’ll flank, and if they don’t, they’ll form a line, circle up with their backs to one another or send one of their number to run and get more guards. When they’re in serious danger—reduced to 4 hp or fewer, or next to another guard who is—their discipline dissolves, but not so much that they’ll run without Disengaging (action) first, unless they themselves are the ones seriously wounded. (The difference between their physical abilities and those of a commoner is significant enough that nonhuman guards will take the direct approach to combat even if commoners of the same race wouldn’t.)
Note that guards don’t even have proficiency in the Intimidation skill. They can yell, “Halt!” but it’s not going to make anyone take them more seriously.
A thug has Multiattack, Pack Tactics and a physical profile that I categorize as “brute”: high-Strength, high-Constitution, melee-tailored. Thugs’ standard combat style is to fight as a gang, getting as close as possible to their targets in order to give one another advantage on their attack rolls. Although they carry crossbows, they’re not especially cut out for ranged combat (for one thing, their Multiattack won’t allow them to reload a crossbow any faster, whereas it does allow them to strike twice per turn with a mace), and being shot at from afar is one of the few things that will make them think twice about a fight. If they can spare the numbers for at least two of them to break off and go after a ranged opponent, they will; but one alone will not, because then that thug would be giving up the Pack Tactics advantage, and they like their fights easy.
A seriously wounded thug (reduced to 12 hp or fewer) will Disengage (action) and retreat, and in the following round that thug’s allies will retreat as well, even if they themselves are not seriously wounded. But this in no way means that they consider the fight over. Thugs are available in almost infinite supply, and they may show up again, looking for a rematch, at any time, in greater numbers than before. (Whatever difficulty the last encounter was, based on the table on page 82 of the Dungeon Master’s Guide, make the next encounter one level more difficult, for as long as it suits your story to keep sending vindictive thugs against your player characters.)
Despite their respectable physical prowess and enthusiasm for fighting, thugs are lazy and would rather get what they want by Intimidation, and this is what they’ll try first. After all, if a victim caves now, he or she is likely to cave again later, and again, and again . . .
The Redbrand Ruffians in The Lost Mine of Phandelver are thugs, given another name for that adventure. They follow the same tactics that thugs normally do.
A veteran is a trained, experienced fighter—a soldier of rank, a captain of the guard, a longtime mercenary or the like. Schooled in warfare, they do know strategy and tactics and will fight accordingly. They fortify their positions, take advantage of terrain and choke points, recognize and try to counter their enemies’ plans, seize opportunities to punish their enemies’ mistakes, Attack against weaker fighters and Dodge and Disengage against stronger ones, and parley when their chances of victory look poor, whether or not fighting has begun yet. A veteran is hardly ever a fanatic; he or she has no interest in fighting to the death. A serious injury (being reduced to 23 hp or fewer) is enough to convince a veteran that surrender or retreat is the wisest course of action.
Most veterans will fight hand-to-hand, either dual-wielding longsword and shortsword or wielding a longsword alone, one-handed with a shield (not mentioned in the Monster Manual—with a shield, the veteran’s armor class is 19, not 17) or two-handed without. But in any given group of veterans—or a group of guards and/or thugs led by a veteran—up to half may wield crossbows and use these either to attack ranged opponents or to provide covering fire for the melee fighters as they move. Covering fire is provided by means of the Ready action, with the trigger condition being an enemy exposing him- or herself to attack a moving melee fighter. A veteran with a ranged weapon, or a ranged fighter under the command of a veteran, will make maximum use of any available cover; he or she will not shoot from a position out in the open. Wherever they may be, they’re aware of what positions afford maximum combat advantage, place themselves on these positions beforehand and stay on them for as long as they can hold out. They can’t be fooled into leaving such a position to chase an opponent. They remain conscious of their purpose and won’t be distracted from it. Also, guards being covered by ranged fighters know better than to move beyond the normal range of those ranged weapons.
One other thing that distinguishes veterans from guards and thugs is their Athletics skill. This has various applications, but the one that stands out to me is its use in grappling: veterans are good at nonlethally subduing an opponent as well as escaping being grappled themselves. Guards, peculiarly, are not especially well-equipped to seize and restrain someone, but veterans are. Also, veterans may flee in the face of an overwhelming threat, but they don’t freeze, as a commoner might.
A knight is a fighter on par with a veteran, equally skilled at melee fighting, slightly less so with ranged weapons, with the Parry reaction and the Leadership action. The Leadership action is similar to the Battle Cry action of the orc war chief, only instead of granting advantage on attack rolls, it grants an additional 1d4—equivalent to a bless spell—on the attack rolls and saving throws of each of its allies within 30 feet. Two other key differences from Battle Cry are (a) that Leadership doesn’t allow the knight to make any attack on the same turn and (b) that it lasts for a full minute, not just one round. To get the most use out of it, then, the knight will use the Leadership action immediately as soon as either combat is imminent or it’s already begun; after that, the knight uses the Attack action exclusively. (Knights wear plate mail—they don’t need to Dodge.) The knight has no good reason to forgo the use of this feature, even at the cost of its own Attack action for one round, because its effects are long-lasting and proportional to the number of troops it commands.
As for the Parry reaction, a knight will use it against whichever of its melee opponents (if it has more than one) does the most average damage per hit. The knight is familiar enough with a variety of weaponry, and enemies, to be able to judge this accurately. For instance, if a knight is being attacked by a paladin with Strength 16 and a longsword (1d8 + 3, or 7.5 hp per hit) and a barbarian with Strength 18 and a greataxe (1d12 + 4, or 10.5 hp per hit), he or she knows to let the paladin’s blows land and parry the barbarian’s. This is a function of experience, not Intelligence.
A knight may be mounted on a warhorse, which in addition to a melee attack with its hooves also has the Trampling Charge feature. A mounted knight will always use this feature, if possible, when first engaging with an enemy. Since the warhorse has a base movement speed of 60 feet, and the charge only requires a 20-foot head start, a knight already in the thick of combat who defeats an opponent will circle away to get at least 20 feet from his or her next target, using the Disengage action if necessary, then go back in with Trampling Charge. He or she will then Attack the same turn if Disengaging wasn’t required or on the next turn if it was. [See note in comments below.]
Next: Shady characters.