If I recall correctly, often times the daughters of kings would be married off to neighboring kings/counts/dukes. Communications between families could have played a role in maintaining alliances (alliances were probably forged initially, at least in times of peace, through marriages of princesses and princes).

In times of war, kings often led their soldiers to battle. Since a neighboring kingdom would not be to keen on an army marching straight into their borders, and a king probably would not leave his army, high ranking nobles, and possibly even clergy men, would conduct negotiations with other kings. The use of bishops, abbots, and other clergymen were especially useful, as it would appear as if an action was sanctioned by the Pope, and by extension, God.