Thucydides (d.395 BC) in his History of Peloponnesian wars, not only gives the details of the war fought between Athens and Sparta but also points out the psychological impact on the people and the change that came about in society. He argues that is was the mutual fear among the warring parties which leads to bloody conflicts.

Wars also create the fear of violent death among the common people. Thucydides further argues that war warps human nature and values such as love and sympathy, while the sense of brotherhood and sacrifice for others disappears and is replaced by self-interest, hostility, hatred and envy. According to him war becomes a tragedy which disintegrates the entire fabric of society and annihilates moral values and social customs.

In the Greco-Persian war, after defeating the Persians, the Greeks became arrogant. Emboldened by victories, Athens became an imperial power and conquered most of the Island of Skyros and established colonies. With the expansion of Athenian power, Sparta became fearful of its imperial designs. This fear led to the Peloponnesian war which not only exhausted their resources but plunged them into disaster and wasted their energies, depriving them of stability. Athens also suffered heavy losses.

People from villages and small towns migrated to the city of Athens for protection. In the absence of basic facilities, most of the people died of disease and starvation. Athens was affected by plague which killed people in large numbers including Pericles (d.429 BC), the leader of Athens. The war and plague deformed the nature of Athenian people who resolved to take care of their own survival at the cost of others.

Thucydides, describing the war and its impact, points out that in the battlefield the bold and courageous were killed while the cowardly and cunning people survived — a characteristic that can be found in all wars.

After defeating their enemies, conquerors slaughtered common people without any sense of mercy or humanitarian feeling. Intoxicated by his triumph, Alexander killed his old comrades and generals on the slightest suspicion. In their desire for glory and grandeur, Caesar, Genghis Khan, Napoleon and other invaders massacred common people establishing the fact that as human beings they had become bereft of emotions.

In the First and the Second World Wars, it was fear of each other in the European powers which entangled them into conflict. One can also see the deformed human nature which led the world to disaster and destruction. In the First World War, the soldiers living in the trenches day and night had lost all feelings of nationalism and patriotism. Crouched in the trenches, sometimes right next to the bodies of their comrades, who had been killed by shelling or snipers, they awaited their death at the hands of the enemy. In the Second World War, countless Jews were killed in gas chambers by the Nazis in the concentration camps in Germany without any remorse or human sentiment. When atom bombs were dropped on Hiroshima and Nagasaki, the pilots hardly realised the disastrous repercussions of the explosions that destroyed the cities and killed the inhabitants.

The warped nature of human beings can also be seen during wars when cunning and crude people took advantage of the misery of the common people by making profits in trading and manipulating the prices of daily-use commodities. They become wealthy and enjoy a luxurious lifestyle while neglecting the suffering and pain of others. In the American civil war they were known as carpetbaggers.

According to Thucydides, every war is a tragedy which leaves behind painful memories of those who were victimised. After the war, it takes time to eliminate fear and to change the deformed human nature. Peace and prosperity are restored with difficulty as is human emotion.