Every species within the animal kingdom knows how to fight. It is instinctual for animals, including humans, to fight for survival, in fact. Diplomacy, however, is a means for man to separate himself from animals by demonstrating that reason and negotiation can achieve a peaceful and happy arrangement for all involved. In this way, diplomacy is a sign of a truly civilized society.
The pen is certainly mightier than the sword. In civil society, how often do you see disagreements settled through force? Such things are illegal first of all, and furthermore, I would argue that most people consider violence counterproductive in civil disagreements. If that is so, why should it not be so in international affairs? Peaceful means are the best ways of resolving disagreements in all but the most dire circumstances.