Heres some info from the official website
WARS AND ALLIANCES
Wars
Wars are formalized hostile relationships between guilds, which allow open combat between the guilds involved in the war. Both sides must agree to the war declaration in order for a state of war to be declared. Click for more details.
Alliances
Alliances are formal protective relationships between guilds, and are managed by the guild designated as the Alliance Leader. Guilds can not have any wars declared when joining an alliance and a guild may only belong to one alliance at a time. Alliances share all wars declared by their Alliance Leader, and only the Alliance leader can declare war or surrender. Other guilds wishing to declare war on any guild in an Alliance must declare war on the Alliance Leader directly. If the Alliance Leader accepts the declaration of war, this will result in a war with the entire Alliance. Allies will highlight the same color as members of your own guild (a light green color by default).
Breaking Alliances
A guild can remove itself from an alliance at any time; however the guild breaking away will not retain any wars that were declared on the alliance. If the Alliance Leader leaves the alliance, a random guild in the alliance becomes the new leader. Only the Alliance Leader can remove other guilds from the Alliance.