Wars allow countries to invade & occupy regions for any reasons
To start a war, a country must do the Declare War law against another country
Once the law is accepted, you may begin attacking the country's regions

During a war, President, Vice presidents & Minister of defense may attack enemy regions by:

A region may only be attacked if it is adjacent to one of your occupied regions. In other words, attacks can only be launched from neighboring regions, you cannot attack regions that are separated by another region
If one of your country's core regions has been occupied by an enemy, you may launch a Liberation Battle to retake it.

Liberation Battles grant a damage bonus, making it easier to reclaim your country's core territory