If no relations exists between two clans, then they are considered neutral towards eachother. There are three possible diplomatic stances. You can be friendly, or neutral, or unfriendly.
Ninja and Kunoichi may act regardless of the diplomatic stance with a clan.
A declaration of war inmediately eliminates all trade routes with that clan.
A clan must be at war with another clan, if its armies wish to attack that clan.
Your clan cannot declare war on an ally. Your clan must break the alliance, before your clan can declare war on that clan.
Wars can end if both sides declare a ceasefire. A ceasefire can be declared if one side sends a ceasefire agreement and the other side accepts that agreement.