Only Alliance Leaders and Co-Leaders can start wars by opening the World War screen and tapping the “Start New War” button. Once they choose the Alliance members that are going to battle, the search for an opposing Alliance will begin. There must be at least 10 members chosen to go to war, and the number of participants must be a multiple of 5. “Unavailable” players may not be chosen to participate in wars. There is no need to wait at this screen; you can return to your town and will be notified when the search is successful. Planning Day will start immediately after an opposing Alliance has been found.

Are there any requirements for going to war?
Your Alliance must have at least 10 members who have built the Command Post to go to war. 

Why are some alliance members unavailable for war?
When Leaders tap the World War button > “Begin New War” they get a list of all available players, and are prompted to choose who they are taking to war. Some Alliance members may display as “Unavailable” because they haven’t built a Command Post, have opted out of participating in wars, or are still in an active war from their previous Alliance. All other players will be able to be toggled in or out.

I changed my mind. How do I cancel my war?
Alliance Leaders can cancel the search for a war by tapping the cancel button on the search screen at anytime. There is no penalty for cancelling the search. If a war has already been started, it cannot be cancelled.

What happens if I leave my Alliance during an active war?
Your war base will remain on the war map if you leave an active war at any point after the search has been started. Leaving a war will not impact it in any way. You are free to join other Alliances or come back to your Alliance and continue participating in the active war. 

If you leave an active war, you will not be able to participate in a war with other Alliances until your current war has been completed. You will still receive any War Loot at your Command Post earned during your current war, even if you are no longer in that Alliance.