Holmes's discussion of combat is longer than in OD&D for two reasons. One, there are no references to Chainmail and everything, right down to how to roll dice, is explained in greater detail. The result is a system that's no more complex than in the LBBs but is more clear. Holmes does note, however, that
The combat tables used by D&D gamers are often extremely complicated. Full tables are given in ADVANCED DUNGEONS & DRAGONS. The tables below are deliberately simplified, but will take some practice to use them with facility. Once the system is mastered, however, players can add whatever modifications they wish.Again, Holmes encourages tinkering with the rules. His "deliberately simplified" combat tables cover only levels 1-3, at which levels all character classes have the same hit probabilities in OD&D, although this differs from AD&D, where fighting men gain better hit chances faster. Holmes also includes a line for a "normal man," something not explicit in OD&D but descended from Chainmail. The numbers on the chart match those in the LBBs.
Holmes retains a strong association between an armor class number and the type of armor it signifies, so AC 7 says "leather armor" in the combat chart itself, a connection AD&D hopelessly muddles with its introduction of overlapping armor types. Unfortunately, the Blue Book isn't entirely consistent on this point.
The "armor class" of humanoid monsters is literally the armor they are wearing (or possibly their skin/hide!). For non-human creatures, however, it is assigned partly on this basis, with strong armor class for scales and shells, and partly on the basis of difficulty to hit. Thus a small fast creature, like a vampire bat, might be hard to hit with a sword and could be assigned "armor class: plate" (AC 3) to indicate this, although its own skin would make it seem more like "armor class: none."Granted, this ambiguity has always been present in D&D, but this passage really lays it bare. Oddly, Holmes uses the phrases "armor class: plate" and "armor class: none," which I rather like, as they show much more clearly that the armor class numbers are not meant to be target numbers but rather numeric signifiers -- shorthand for ease of communication.
Holmes's monster attack tables differ from those in OD&D. The hit dice categories are slightly different, especially at the low end. For example, the LBBs start the chart at "Up to 1" hit dice, while Holmes starts it at "up to 1+1." It's a small thing but a possibly noteworthy one.
Simple rules for poisoned weapons are introduced, with differing saves depending on the effect of the poison. The rules also note that "It is recommended that the Dungeon Master not allow players to make use of poisoned weapons in all but extreme situations." Flaming oil is explicitly treated as a weapon as well. There are details of how much area burning oil covers, how much damage it does, and how long it burns before extinguishing. More interesting is that there's a system presented for how to throw oil, with the number needed on a 1D20 roll not being connected to the target's armor class but rather its size. Using oil is a two-stage process, requiring first a hit with the oil and a separate hit with a flaming object, such as a torch or a lantern, to ignite it. Burning oil is effective against corporeal undead, although those immune to normal weapons take only half damage from it. Of course, holy water works exactly like burning oil against all undead, except that it (presumably) doesn't require a second roll to ignite it and it does full damage against even those immune to normal weapons.
Missile combat is fairly straightforward, with short ranged attacks getting a bonus to hit and long ranged getting a penalty. Cover is treated briefly and it's noted that characters cannot safely fire into melee because of the probability of hitting friendly characters.
Holmes follows OD&D in noting that a magic weapon's bonus applies only to hit, not damage, as additional damage is among those "other powers" a magic weapon might possess. Meanwhile, bonuses for magic armor and shields subtract from the attacker's roll rather than changing the armor class of the defender. It's a small point perhaps, but it's consonant with the LBBs' approach and is further evidence that armor class was originally intended to be static and closely tied to a single armor type.
As noted earlier, melee rounds are 10 seconds long. Movement during combat is limited to 20 feet per round for an unarmored man and 10 feet per round for an armored one. Daggers grant two attacks per round, while two-handed swords, battle axes, halberds, flails, morning stars, and most polearms can only be used once every other round. Light crossbows likewise operate only once every other round and heavy crossbows take "twice as long to load and fire." Monsters, on the other hand, can use their full routine of attacks each round.
Initiative is determined by Dexterity, with highest Dexterity going first. However, if the Dexterity scores of two combatants are"within 1 or 2 points of each, a 6-sided die is rolled for each opponent and the higher scorer gains initiative." Surprise is mentioned as granting initiative but there are no explicit rules for surprise. Attacking a fleeing target grants a +2 bonus to hit and the target does not benefit from wearing a shield. Holmes introduces rules for parrying. A character may elect not to attack but to parry an incoming attack, which imposes a penalty of -2 on his attacker's roll. If the attack still hit by rolling exactly the number needed, the parrying weapon is broken but no damage is inflicted. It takes a round to draw a new weapon. Holmes also grants a +2 bonus to attack any opponent withdrawing from combat. He mentions the possibility of surrendering but provides no rules to handle it beyond referee judgment.
The Blue Book provides two combat examples, a short one between a fighting man, Bruno the Battler, and a goblin. It's a very straightforward combat without anything unusual in it. The second example pits a party against six large spiders. Among the characters are Bruno the Battler in a return engagement, another fighting man named Mogo the Mighty, a magic-user named Malchor, and a cleric called Clarissa. Bruno "dies a horrible death" because of a failed save versus poison, alas.
This last example illustrates several things. When there is time, or when a magic-user says he is getting a spell ready, magic spells go off first. This is followed by any missile fire, if the distance to the monsters permits, and then melee is joined, after which no missile fire is permitted because of the danger of hitting friendly forces. If a magic-user is not involved in the melee he can get another spell off after 1 or more round have gone by. If he is personally attacked he can't concentrate to use his magic but must draw his dagger and defend his skin! However, if the magic-user had some magical device -- such as a wand or a staff -- it could be used in lieu of the dagger as an attack weapon.And that pretty well sums up Blue Book combat, which is largely in line with the LBBs mechanically but much more clearly presented and simplified, adding only a few rules, such as parrying and Dexterity-based initiative, that seem to represent house rules employed by Holmes.