Diplomacy evolved initially to deal with problems in the relationships between countries.
Instead of leaders infuriating each other and making decisions in the heat of the moment, they learnt to send emissaries who could state things in less inflammatory ways, who wouldn't take the issues so personally, who would be more patient.

The start of diplomacy

A diplomat is serene in the face of bad behavior, such as a sudden loss of temper. 
They don't take a wild accusation or a mean remark personally but understand that the person who bangs a fist of the table may be worried, frightened, or just enthusiastic - conditions that should instead invite sympathy.

Bad behavior

Within a negotiation with someone, there is often a request that they change in some way. 
A diplomat knows that it is futile to state the call to change too directly as many insist on having their way. Behind the arguing may lie a need for appreciation and esteem.

Direct confrontation

A diplomat understands that there are moments to sidestep direct engagement. They don't teach a lesson whenever it might first or most apply; they wait until it has the best chance of being heard. 
They can disarm difficult people by reacting in unexpected ways. They might nod in partial agreement to unfair criticism and declare that they've often said such things to themselves. In the face of a tirade, instead of getting defensive, the diplomatic person might suggest some lunch.

Sidestepping direct engagement

A diplomat has given up on the ideal out of mature readiness to see compromise as a necessary requirement in an imperfect world.
The diplomat may be polite but is willing to deliver bits of bad news. We should say that they're fired, that their pet project isn't going ahead, but instead pretend there is a glimmer of hope. Real niceness means helping the people we are going to disappoint to adjust as best we can to reality. The diplomatic person administers a clean blow and kills off the torture of hope, accepting the frustration that's likely to come their way.

The realist

Diplomats know that fear holds people back and therefore offer love and reassurance.
They know too when recommending change, they are not speaking from a position of perfection. For a recommendation not to sound like mere criticism, the diplomats know to admit to their own shortcomings: "And I am, of course, entirely mad as well…’’

Conquering fear

Diplomats know the intensity with which humans crave respect. Diplomats take the time to show that they have bothered to see how things look through the other person's eyes.
Diplomats perceive that people want to feel heard as much as they want to win their case. Therefore, diplomats put effort into securing the overall relationship's health so that smaller points can be won along the way.

Craving respect

Diplomacy is the art of promoting an idea or cause without unnecessarily inflaming passions.
It involves an understanding of the many parts of human nature that can lead to strife and a commitment to handle these with foresight and grace.

Diplomacy is an art

How to Be Diplomatic - Articles from The School of Life, a gathering of the best ideas around wisdom and emotional intelligence.