Declarations of War
The declare war option is used to officially open armed hostilities with another clan, as opposed to simply marching into their territory to stake your claim. Never declare war without first considering the consequences, as an ill-prepared assault could have consequences resulting in your reduction to vassal status or worse, the annihilation of your clan. Note that when making your declaration the check box asking allies to join you is defaulted to yes. Your allies will assess the situation and may agree to declare war, but might just as easily break off your alliance, leaving you to fend for yourself.
Joining a War
At any time during a conflict, the join war option can be used to request that an ally declares war on a clan you are already fighting against. This means you can look for new allies to help if the war is not necessarily going your way.