The use of diplomatic commands will prove vital to help place other forces in Debt to you, or enable you to receive a recommendation from another officer to increase your Negotiation Rate when engaging in diplomacy.
Debt can be incurred by sending gold and gifts to other forces or by responding to requests from them. The more Debt that is incurred, the more likely you are to succeed in your negotiations. If your officer is selected as a messenger, you can use techniques such as Endorse or Bargain to bring about Debates that can increase your Negotiation Rate and allow you to successfully complete more difficult negotiations.

You can check your diplomatic relations with other forces on the Planning screen.

On the negotiation partner selection screen, you can check the state of your debt with each force.

The results of your negotiations depend on the Intelligence of your officer or Abilities such as Negotiate. Additionally, you can use Debt to greatly increase your chances of success, so be sure to make all of the necessary preparations before the negotiations begin.

It is important to find officers you share a strong relationship with on the Chart.

You can use the Endorse command to gain the recommendation of an officer you have a certain level of Rapport with.

To increase the Negotiation Rate even further, you can use the Bargain command to engage in a Debate with a minister of the opposing force.

An alliance has been formed with the other force. You can use Debt and the Negotiation Rate to lead your diplomatic negotiations to success.

※Images taken from in-development Windows® system version.