The final aspect of gameplay is for many players also the most important one: combat. If diplomacy fails, or if you're just feeling like kicking some arse, you can declare war on an opponent and take what you want from him/her by force. To do so, you need to build an army of Units and use those to attack enemy Units and Cities, or to pillage his/her Improvements and as such damage his/her economy. But even if you don't want to go to war, you'll still need to build an army: if someone else declares war to you, you'll need to be able to defend yourself. And even if other nations don't declare war, Barbarians might invade you. Posting sentry guards along your borders and fortifying Units inside your Cities to guard them is a wise precaution. When expanding your empire peacefully, you will need to explore the lands around you to find suitable locations to build new Cities, and you'll need to send out Settlers to actually found those Cities (along with escorts to protect them). You need to build Units for these purposes as well. And of course, in order to expand to other continents (either peacefully or aggressively), you will need a navy.

Units in Civilization IV function by and large in the same way as in Civ3, but there are some notable differences as well. Most Units still have the basic commands: move, attack, fortify, pillage, disband, sentry, (un)board transport, skip turn. Civilization IV offers one additional standard command: explore, which lets the Unit automatically explore the map.

Another minor change is that Units now get a bigger defensive bonus the longer they are fortified. After two turns fortified, the bonus is 10%, after five turns it's 25%. That is also the maximum fortification bonus.

A bigger difference is in Unit stats: in Civ3 Units had both an attack and a defense value, in Civ4 they only have one generic Strength value. This Strength value is used both to determine the chance that a Unit hits its opponent in combat (similar to the attack/defense value in previous Civ games) and the amount of damage that is done when a hit is scored (similar to firepower in Civ2). This makes the classic Spearman vs Tank problem much less of an issue, as a Spearman not only has a much smaller chance of doing damage, it also does much less damage.

Despite the single Strength value there is still plenty of variation in the combat model, as many Units have much more unique special abilities: Spearmen have a bonus versus Mounted Units, Archers have a bonus defending Cities. Chariots and Cavalry don't get defensive bonuses, but they do have a chance to withdraw from a battle if it's going badly. With this system, every Unit has a specific role in the game and every unit has its own counter. Just building a huge army of the same type of Unit is not a very good strategy, as that makes it very easy for an opponent to counter it. So players are encouraged to build varied armies with Units of different types, in other words to use combined arms.

But it doesn't end there. Another major way in which the combat system has been refined and diversified compared to earlier games in the genre, is through Promotions. As a Unit fights in battles, it builds up Experience Points. The tougher the battle, the more experience it yields. And every time the number of Experience Points surpasses a certain threshold, the Unit gains a Promotion. There are many different types of Promotions from which a player can choose, which allows players to really specialise the abilities of their Units and to create a varied army with many unique Units.

Some examples of Promotions are: a generic Strength bonus, a bonus versus a particular type of Unit, a bonus attacking Cities, the ability to use enemy Roads, the ability to heal other Units, a movement bonus over Forest or Hills, etc. In total there are 41 Promotions in the game, but not all are available to all Units all the time [see image on the left]. For a full overview of all Promotions, see the Civilopedia.

In Civilization IV, the traditional health bar is gone. Every Unit consists of 3 men and each of these men represents a hitpoint (although the exact number varies in screenshots). When a Unit receives damage, the men will die off to represent the damage taken. When no more men are left, the Unit is dead. Units also all belong to a particular Unit Category, such as Melee, Archery or Mounted. This is because the special abilities system for both static Unit properties and for Promotions makes frequent use of this.

There are less different types of Units in Civilization IV than there were in Civ3 and the total number of Units in the game should be lower as well, but this makes every individual Unit more important and highlights the new Promotions system. A few new Unit types have been introduced though, such as Animal attackers (Lions, Bears, Panthers, Wolfs) for Barbarians and (non-Unique) Grenadiers, Horse Archers and War Elephants for regular Civs. There is a total of 83 different Unit types. A breakdown of all Units in the gamecan be found in the Civilopedia.

Army management has also been revamped considerably compared to the games' predecessor. Armies that fight together as one are completely gone. Armies still exist but they now only serve to move Units as a group. With a single click an army, or group, of multiple Units can be formed and these can then be moved around as if they were a single Unit -- even if they are of different Unit types. The management of groups of Units like this has been greatly improved compared to Civ3. There is no limit to the size of an army in Civilization IV.

[texthighlight]Civilization IV: Warlords[texthighlight] will introduce another new combat concept, namely that of the Warlord units. It's not yet known how it will work.

All-in-all, combat in Civilization IV is neither more nor less complicated than combat in Civ3, just very different.