You can declare war on any regime that not has DipBlock set on it.

Declaring war can cost rulevar 818 PP to do.

You can declare war on any regime you are either at peace with or allied with.

You will also automaticly declare war on all allies of the regime your declaring war on.

Combat may be initiated in a myriad of hexes where now hostile units share the same hex. Also ZOC conquest will be immediatly executed taking into account the new situation.