How Combat Works (Basic D&D)
From D&D Wiki
How Combat Works
In Basic D&D, combat occurs in a series of rounds of 10 seconds each. However, it uses a much different and less-well defined system of combat, with the correct information being scattered across pages or rulebooks. It tends to require house ruling for anything more detailed unless you have access to all the necessary rules.
- At the start of each combat round, each group (or individual) rolls for initiative.
- Characters or groups with the highest initiative act first. If two players or groups have the same number, the actions occur at the same time.
- At the start of each combatant's action, morale is checked if applicable (some NPCs and monsters need morale checks.)
- Combatants acting may perform the following:
- Missile combat
- Spell casting
- Hand-to-hand combat
The Combat Round
Basic D&D defines a combat round as 10 seconds. In this round, a combatant is permitted one action, as defined by the rules.
For combat, initiative is rolled. If the game follows the simple rule of initiative, roll a 1d6 for each side. For individual initiative, roll a 1d6 for each individual, adding the initiative modifier from dexterity.
In addition to the die roll, some effects cause combatants to either win or lose initiative. These combatants are placed at either the beginning or end of the combat round progression, and are in their own group for determining initiative. The most common cause is using a two-handed weapon, which causes a loss of initiative, making no exceptions for a staff-wielding magic user trying to cast a spell.
Initiative is also lost if the combatant has to draw a weapon.
When a creature's turn is available through initiative, actions in a given initiative slot are performed according to priority. Movement takes place first, followed by missile combat, followed by magic, then by hand-to-hand. Once the actions are resolved for an initiative slot, the next turns come up.
Some enemies or NPCs will try to disengage from combat. This is not part of the actions, but is used to determine behavior of NPCs.
It is generally checked by a 2d6 roll against the monster's morale score:
- When a group tries to evade an encounter.
- Every five combat rounds while chasing.
- During combat, when a targeted creature takes 1 point of damage.
- During combat, when a creature is reduced to 1/4th of its starting hitpoints
- During combat, when the first death on each side takes place (regardless of which side gets the fatality)
- During combat, when only half of the creatures of their group is free to act
- When a creature is affected to a weapon master's despair effect
- When a creature is affected by a magical item or spell that requires a morale check.
- When a PC's retainer is ordered into danger while the employer remains in safer surroundings.
The following bonuses apply:
- If monsters have killed at least one enemy without taking casualties themselves, +2 bonus.
- If monsters have killed at least one enemy but have taken losses, +1 bonus.
- If monsters are on the receiving end of magic and have no magical abilities, -1 penalty.
- Other optional penalties may be applied, but the total bonus or penalty should be within +2 and -2.
The following movement options are possible if outside of hand-to-hand combat:
- A combatant may move the full encounter speed.
- A combatant may run at three times normal encounter speed.
If engaged in hand-to-hand combat:
- A combatant may move 5' without taking a combat action, such as after attacking. 
- A combatant may perform a fighting withdrawal. This allows moving up to half the encounter speed, although other sources say the limit is 5 feet. After moving from a fighting withdrawal, the combatant may then attack any enemy that moves within range, at the end of their movement phase but before said enemy can make a melee attack.
- A combatant may retreat from hand-to-hand combat. If the combatant is later attacked that round, the attacker gets a +2 bonus to hit, and ignores the AC bonus from shields (as if being attacked from behind).
The following missile attacks are possible:
- An item may be thrown. If a combatant has multiple attacks, he can throw a weapon for any attack he makes in a round. The attack uses dexterity modifier to hit, and strength modifier for damage.
- A missile weapon may be fired. The attack uses dexterity modifier to hit. However, missile-fired weapons are ineffective against targets within hand-to-hand combat, or against mobile targets within 5' unless the target can't move. 
When casting a spell, you must declare the spell being cast before initiative is rolled. If the spell caster is able to cast the spell as soon as it is time for the caster to act, the spell is cast successfully. If the caster is hit by an attack between declaring the spell and casting it, the spell fails and is lost.
The following attacks are possible:
- The combatant may make an attack. If not engaged in melee, they may move up to their encounter speed as described above, then attack.
- A fighter, dwarf or elf may make a lance attack. This requires being mounted and running at least 20 yards towards the target. It inflicts double damage on a successful hit (before adjustments).
- A combatant on foot that is carrying either a spear, pike, sword shield, or lance can set the weapon versus charge. This action must be declared before hand-to-hand combat, and if the enemy performs a charge or lance attack or runs at least 20 yards before making an attack, causes double damage (before damage modifiers). The target must be running or trying a lance attack for this tactic to work.
- A smash attack may be performed by level 9 fighters, mystics, and appropriate-level demihumans. It takes a -5 penalty to hit, but adds the entire strength score to damage.
- A parry does not attack, but causes a -4 penalty for incoming melee and thrown attacks. Missile attacks are unaffected.
- A disarm attack, available to level 9 fighters, mystics, and appropriate-level demihumans. The attack inflicts no damage but causes the target to drop the weapon.
If a combatant has to draw a weapon, the combatant will lose initiative. 
Unarmed combatants normally do 1 point of damage, plus strength modifiers (not counting natural attacks that some monsters have). There are optional rules for striking and wrestling, which are not described here.
- Rules Cyclopedia, page 66
- Rules Cyclopedia, page 102
- Dungeons & Dragons, Expert Rulebook 1981 edition, page X24 and Dungeons & Dragons, Basic Players Manual, page 60
- Rules Cyclopedia, page 104
- D&D Basic Players Manual page 59, or Rules Cyclopedia page 108
- Dungeons & Dragons, Expert Rulebook 1981 edition, page X11
- rules Cyclopedia, page 103
- Rules Cyclopedia, page 105