Fighting on a tactical battlefield gives you the best chance for a crushing victory over your enemies. This option may be turned off using the preferences screen at any time during you turn. Once you enter the tactical battlefield, you are committed to fighting there, but you may choose to let your Defense Minister handle the battle for you.
Once both sides have deployed their forces the battle is fought in a sequence of unit turns based on the initiative rating of the individual units. This means that several of your units might act before any of the enemy regiments; or, that the enemy forces might all act before you can. Generally, cavalry acts before infantry, and infantry acts before artillery. However, these rules are modified by the experience rating of the regiments and the qualities of the leaders on each side.