An army can attack another army, a port or a settlement. An attack on a settlement will result in a siege taking place. If the target is further away than your army's movement extent will allow, it will take more than one turn to reach it and launch an attack. Once an army has reached its target the battle panel will be displayed, prompting you to fight on the battlefield, resolve the battle automatically, withdraw or lay siege (when appropriate).
You can attack a fleet belonging to another faction when it is within your zone of control by right-clicking on it. If the target is further away than your fleet's movement extent will allow, it will take more than one turn to reach it and launch an attack. Once a fleet has reached its target the battle panel will be displayed, prompting you to fight, resolve the battle automatically or withdraw.
In Total War: ROME II, when you capture a faction’s last settlement they are not automatically destroyed. Any remaining forces will immediately begin suffering attrition and will dedicate their time to trying to recapture a region in order to re-establish themselves as a viable faction.
The battle panel presents you with several options for resolving the upcoming battle - fighting on the battlefield or seascape, resolving the battle automatically via a comparison of each force's statistics and withdrawing from the fight. If you withdraw when attacked there is a good chance that the attacking force will attack you again. If you have attacked a provincial capital there will also be options to continue the siege, to break it off, and to construct siege engines on-site for assaulting enemy settlements. Note that siege engines take a number of turns to complete before use. If you have more than twenty units they will automatically be fielded as a single force. If you wish to field twenty units and have any others appear as reinforcements you must uncheck the box on the battle panel.
When auto-resolving a battle, you can give high level instructions to your generals on how to fight the upcoming battle by selecting a battle stance first. Comparing the composition of each army before choosing the stance best suited for the situation ensures a much better result. The balanced battle stance enables armies to retreat from the battle and, unlike other stances, doesn’t give bonuses or penalties to either the attacker or defender. The following stances are possible:
The adoption of aggressive stance ensures your units try to rush the enemy and won’t let them escape the battle. When using it, the following bonuses and penalties are applied: cavalry and other mounted units receive an attack bonus, whilst enemy spear infantry receive a bonus when fighting against them; missile units on both sides have their effectiveness reduced; your units get a global attack bonus, whilst the enemy gets a defence bonus; retreat is not an option for either side, so expect casualties to be high.
Additionally, in sieges, if attacking, the more siege engines you have (towers, ladders, etc.), the more effective your spear and melee infantry become; if defending, the walls offer less protection, but the more spear and melee infantry units you have, the smaller the enemy’s bonuses for their siege artillery (onagers, ballistas, etc.)
Adopting protective stance ensures your units take a defensive position, advancing slowly in formation. They will attempt to retreat if things get out of control and won’t try to chase down retreating units to avoid getting enemy tricks and ruses. When using it, the following bonuses and penalties are applied: your spear infantry receives a bonus when fighting against cavalry, whilst mounted enemies get an attack bonus; missile units on both sides have their effectiveness increased; your units get a global defence bonus, whilst the enemy gets an attack bonus; forces will attempt to retreat if they suffer considerable losses.
Additionally, in sieges, if attacking, the more siege artillery you have (onagers, ballistas, etc.), the more effective your spear and melee infantry will become; if defending, the more spear and melee infantry you have, the smaller the enemy’s bonus for their siege engines (towers, ladders, etc.)