An Emissary can only make a pact with the Daimyo of a clan himself, not his generals or heirs, and it can sometimes take a while to find him as your Emissary can only move one province per turn. Once you've located him and made an offer, a reply will come back the following turn. If the Daimyo agrees to the alliance, then your Emissary gains honour which may help him make another alliance. Opponents may want to assassinate your Emissary to stop you making pacts or to increase their Ninja's honour on an easy target, and the Daimyo you are allied to may want to assassinate your Emissary to prevent you from spying on him - even if he wants to keep the alliance. A Daimyo may refuse an alliance because you are at war with one of his current allies or he may ask you to attack someone he is at war with before he agrees.
Cavalry come to battle.
Find out more about the dirty tricks brigade - Ninja and Shinobi.