If you are a Daimyo then you can have other clans as vassals. You can have no more vassals than you own districts. You can only vassalize other Daimyo who have a lower title than you have.
If you do not have relations with another clan, you are considered to be neutral towards that clan. There are three possible diplomatic stances. You can be friendly, or neutral, or unfriendly.
You must be at war with a clan to be able to attack that clan with your armies. Ninja and Kunoichi may be used at any time and regardless of the diplomatic stance with a clan.
You cannot declare war on your liege, vassal or ally, or any clan with which you have a non-aggression pact. You must break your allegiance to a liege, or fire your vassal, or divorce the women of that clan, or annull the non-aggression pact, if you wish to declare war on that clan.
Wars can be ended if both sides declare peace. Peace can be declared if one side sends a ceasefire agreement and the other side accepts that agreement.
If you marry a woman of a different clan to a man of your clan, then the two clans who have brokered the marriage are allied for as long as the marriage exists, that means that both the husband and wife have to be alive.
A married woman moves to the clan of her husband. The main advantage of an alliance is that married women may give birth to children and so they provide offspring for their husband's clan. These offspring may become the future generals of that clan.
A male character may have concubines, but may not marry more than four women. While a female character may have only one husband.
You cannot propose a marriage between two characters who are blood relatives (i.e. they have the same father or mother).
The effects of marriage are automatically cancelled, when either the husband or wife dies. A wife will commit seppuku (ritual suicide), if her husband dies in battle, or is assassinated by a Kunoichi.
Common Marriage
Some of your male clan members may in some instances marry a female Samurai of a minor clan as a last resort, if you are unable to broker a marriage with other clans through diplomacy. These lesser clans are locally established NPC clans and have to appeased with presents. A common marriage cannot end up in divorce. Presents cost 400 koku. The chance to broker a marriage with a minor clan depends on the level of the local castle, the better the castle, the greater the odds.
It is possible to divorce, but divorce is not recommended. Divorce means you gain a penalty of 10 reputation points. Another disadvantage of the divorce is that children may not be born out of wedlock.
It is possible to be the liege of another clan. A liege may give land to his or her vassal. A liege loses 1 extra reputation point each year for every vassal he or she has.
Both the liege and the vassal may cancel the bond that exists between them, but whoever cancels the bond gains a penalty of 10 reputation points.
You may also offer a non-aggression pact to a clan. Clans who have a pact between them, cannot attack eachother for as long as the pact is in place.
A pact has a duration of 7 years, after which the pact automatically expires. Should one of the clans renounce the pact before the duration is over, then whoever cancels the pact gains a penalty of 10 reputation points.
Bad reputation can be gained through: divorce, renouncing a non-aggression pact prior to the expiration year, and renouncing a liege or vassal.
By default you lose 2 reputation points every year. Reputation can range between 0 and 100 points.
For every point of bad reputation that you have, you lose 1 percent of your total income. So it is best to avoid gaining too much reputation.