Thucydides and Herodotus discuss many different causes of conflicts among individuals and nations. Three are the gods, human actions, and human ideas.
Herodotus, like Homer, often shows the gods as a factor contributing to human conflicts, but Thucydides does not.
Another major cause of war is self-defense. When you are attacked, you will go to war to defend yourself (e.g. Greeks defending themselves against Persians).
In Thucydides, a major cause of the Peloponnesian wars was Sparta's fear of Athenian power. Thus fear is another cause of war.
In the Melian debate, Cleon argues that Athens needs to deal harshly with Melos to prevent allies from rebelling; thus conflicts can be caused from the desire to send a message to a third party.
Injustice and arrogance can cause conflict. Athens' high-handed behaviour in the Delian league, according to Thucydides, was another cause of the Peloponnesian wars.