Abstract
Thucydides shows the unchangeable laws or so-called destinies in politics in several respects through his deciptions and comments in The History of the Peloponnesian War.
According to Thucydides，the people own the similar characteristics although they have different rights in different political systems. Firstly，they are oversuspicious. Secondly，they are irrational. They are not reasonable；they are influenced by speeches；they are extremists；they easily lose their confidence；they are easily deceived and so on. Thirdly，they have behaved capriciously. Additionally，people have some other characteristics，for example，they are always unsatisfiable，manipulated and irresponsible，unpredictable changes in policy and so on.
Party faction is a universal phenomenon in Thucydides’ accounts. The struggles in parties are very cruel. The origin of results in such cruelty is many-sided，which including personal rapacity，ambition，and their fierce and crazy motions. So，the source of the struggles in parties and their cruelty are from humanity. The evil consequence is not urban blight，but the moral degenerating of humanity. It is an ordinary rule that the parties struggles introduce external force and incur external force. Among them，introducing external force is the first rule and incurring external force is the secondary rule. Beside these，there is a special rule of party factions. It is the phenomenon of inner hidden traitors. This phenomenon is a variety of the struggles of parties.
According to Thucydides，the politicians can be classified into three grades according to their personal political capacity and virtue，and the first priority is given to virtue. In addition，there are some politicians below these three grades. There are always such four grades of politicians in a political system. Therefore，different models of politicians in Thucydides have universal significance. In works of Thucydides，Pericles is the only one of the first grade of politicians. The typical representation of the second grade of politician is Nicias and Alcibiades is in the third grade. Pausanias and Phrynichus is representation below these three grades. There are relations between politicians and nations，even live or die of a polis.
Thucydides mentioned such political systems as monarchy，dictatorship，aristocratic system，oligarchy and democracy. Individuals are insignificant for democratic system. A politician in democratic system can not content with the system and an outstanding person always can not be tolerated by the system. It is inevitable that the tension between individuals and the city exists in this sort of system. Thucydides simply mentioned the monarchy and the dictator is not a good system. Oligarchic politics oppresses the people；inner conflicts easily occur in oligarchies；oligarchy betrays the countries. Oligarchy also owns other defects. All these are the badness of oligarchy and embody that oligarchy is not the ideal political system. At the same time，the pure democratic system is also not enough. It is necessary that some new elements join in democratic politics and conquer the defects of democracy. The mixed system which is founded upon democratic politics and mixes the elements of monarchy and aristocracy is probably the ideal system.
According to Thucydides，there are friendship relations，fearing relations，power relations，interest relations and honor relations among countries. The fundamental one is interest relations. It is the essence of the relations among countries. Such essence is typically reflected in the change of their relations.
There are two respects can be found in the work of Thucydides，which concern the relations between domestic and diplomacy affairs. One is that domestic affairs affect diplomacy；another is that diplomacy affects domestic affairs. As well as，national characteristics seriously affect diplomacy. There are very different situations when different national characteristics acting on diplomacy.
According to Thucydides，human beings are limited by several factors. This is “the limitation of humanity”. It is embodied in the restriction upon man’s choice and action from human nature，fortuity，fate and divinities. The limitation of humanity is also the limitation of the sphere of human beings’ behaviors. Among them，human nature is the general key to comprehend the Greek significance and the universal significance of Thucydides’ work. It is just the unchangeable human nature that turns the rules described by Thucydides through the Peloponnesian War into universal rules，among which the rule of the unchangeability of humanity and the rule of the necessarily deterioration of humanity are closely related to politics.
Key Words：Thucydides；The History of the Peloponnesian War；Politics；Destiny（Regulation）