Unlike as in the fifth century was warfare mostly a matter for mercenaries in the fourth century BC. This changed the character of the wars as it lead to more specialisation (cavalry and light armed footmen were used more and more), but artillery and fortification were also more developed. Most of all changed the character of the wars from a number of large scale battles towards a guerrilla warfare. Losing had become much worse as the warriors fought no longer for ideals, but for money and loot.
Mercenaries as tyrants.
The reasons for the use of mercenaries at a large scale were the bad economical situation, and the big social contrast in big parts of Hellas. During the Archaic period mercenaries were also used, but after that mercenaries could not find any work in Hellas until the final phase of the Peloponessian war. The social and political consequences of the increasing usage of mercenaries were destructive. A captain of a group of mercenaries could permit himself more liberty than a normal commander: he did not have to justify his against against citizens, as long has he paid his men he could do whatever he wanted. The Spartan Lysander was able to influence the politics of Sparta thanks to his own private army.
Every now and then a mercenary managed to become tyrant of a city with the help of an army of other mercenaries. Such a sudden change in the balance of power was mostly accompanied with a partial or full social revolt, remission of all debts and re-allocation of the land, and expel of the local upper classes. These exiles often attempted to regain their power with another army of mercenaries. An example of this fourth century tyranny is Dionysos I who managed to become tyrant of Syracuse in 405. His leadership was very successful as he turned it into the strongest fortification of the world. The events after his death are also representative for the chaos caused by the constant changes of power.
The Greeks were aware that constant war between eachother, and constant changes of power in eachother structures were destructive for all the parties involved. This explains the sudden interest for fortifications of the cities, but it also explains the clause in the charter of foundation of the Corinthian League that says that each member respects and guarantees the form of government of the other members. Before this several attempts had been made to end the rivalry among the Greek Poleis. After the King's Peace in 386 several peace treaties were signed with a list of general rules for relationships between the independent cities. Such a peace treaty was called a koine eirene, or universal peace, as all cities were invited, or forced, to sign the contract. Nevertheless did they have almost no effect on the situation.
The Athenian orator Isocrates proposed another solution: the unity of all Greeks, or pan-hellenism. In his vision this unity could not be achieved with universal treaties as a leader was needed to form and maintain this feeling of unity. The unity also should not be a goal itself, but a result of a cooperative termination of the source of all evil: the contrast between poor and rich. This had to be achieved with an expedition against Persia, with as main goal permanent occupation and colonisation of Asia Minor. Isocrates pointed out the paradoxical fact that after 386 the military superior Greeks were totally dominated by the Persian empire because of their internal discord.
In 380 Isocrates proposed that Athens should become the needed leader because of its experience and history. In 346 he still tried to convince people of his ideas, but now he said that Philip of Macedon should be the one to create unity among the Greeks. With the founding of the Corinthian League in 337 he indeed fulfilled the role which was given to him by Isocrates. The publications of Isocrates furthermore provided the Greeks with the ideology to join in the attack of Macedon on the Persian empire.