The attack view is used to launch an attack on barbarian (i.e., non-Roman). Beneath the name of the province you will find province information about the owner (enemy empire) and the population of the province.
The Enemy Info displays the number and type of enemy units. It also displays the base combat value of the unit and the combat bonus the enemy unit will have when you attack it. Enemy units receiver a +1 combat bonus if your attack will be across sea and +2 if your attack is against a civilized province (cumulative).
Below the Enemy Info are displayed your Activated Legions and Auxilia. In order to attack, you must have activated units available. If you don't have any, you must recruit units or activate garrison units; this can be done from the "Unit" view.
The view provides information about the number of units available, their combat value (10 for Legions and 5 for Auxilia) and their chance of success if they are used to attack the enemy unit.
To launch an attack, press the image button containing the picture of the unit. A popup will appear to inform you of the result of the attack. If you do not have any of those units available (or the target province has been conquered), the unit button will be disabled.
Pressing the close button or the back button will return you to the map view.