Alliance: two clans can ally ("Clan" tab → Alliance). Pick the clan, optionally type a reason (Enter) and press "Send request". The request is valid for 24 h; the other clan gets it in the mailbox together with the reason and accepts or declines it in the clan tab under Alliance. Your own pending request is shown there with the time left.
Allies have no friendly fire, a shared chat (/ally message) and don't fight each other over territories. The alliance map shows all alliances.
Clan war: declare war on another clan with /war clanname – only the leader or members with the "War" right may do this (Clan centre → Rights). For the duration of the war all kills between you count; the clan with more kills wins. /war shows the score; follow it live in the clan centre under "War live".
Clan bounty: place a bounty on a whole clan in the clan tab – also straight from the clan bank ("Bank" right). Every kill on a member pays out a share; allied clans can't get a bounty.
Commands
/ally Nachricht/war Clanname/war