Thucydides, who fought in the war, famously said: “What made the war inevitable was the growth of Athenian power and the fear which this caused in Sparta.”

Yet Athens’ economic power surpassed that of Sparta long before the war. The aggressive expansion of its alliance system, the Delian League, deeply frustrated Sparta but did not make the conflict inevitable.

Pericles’ insistence on building the fortified corridors from Athens to its naval port of Piraeus doomed the relationship. The walls stretched about 7km (4.3 miles), which would sound unimpressive in contemporary China but were considered revolutionary among Greek city states.

As long as Athens remained unwalled, Sparta – with full confidence in the prowess of its hoplite army – believed that it could prevail when push came to shove and, therefore, saw no urgency in military action. However, the walls significantly diminished Sparta’s military advantages.

Sparta and its allies complained that the construction of the Long Walls had changed the strategic calculus in Greece, giving one state an unfair immunity from traditional pitched battles. No wonder Sparta hired flute players to mark the destruction of the walls after three decades of bitter struggle.

Ironically, the walls also played a decisive role in Athens’ downfall. Wisely deciding that it would be suicidal to fight the Spartans in the open, Pericles pulled most Athenians behind the walls. The city became grossly overcrowded. In the absence of modern sanitation, it became a breeding ground for plagues.

Eventually a severe plague struck Athens. In the end, far more Athenians succumbed to the virus than to Sparta’s spears, including Pericles and his two sons.

Thucydides himself contracted the plague and barely survived, but many of his friends died after suffering the excruciating pain caused by the disease.

This undoubtedly helped to shape the historian’s fatalist view of world affairs. But for us today, the story is a reminder of the importance of mutual security over unilateral security.

Athens could argue that the Long Walls were defensive by nature. Yet to Sparta and its allies, the move exposed Athens’ wish to seek dominance in Greece and made it clear that it was not interested in diplomacy.

This might sound absurd at first, but in fact, it was the truth. Athens practised a form of “democracy” – if you don’t count the over 80 per cent of the population who had no right to vote – but was imperialistic in its foreign policy. It actively sought to bring about regime change in other states through sanctions, instigation, and even direct military interference.

Sparta, which practised a rigid oligarchical system, was conservative and introspective. Before the Peloponnesian war, it was not interested in the domestic politics of others and preferred to be left alone. It headed the Peloponnesian League but was a passive leader.

It was, however, deeply suspicious of Athens’ aggressive policy of installing democracy in other Greek city states. Athens’ foreign policy indeed grew more intrusive over time.

By maximising Athens’ security, Pericles increased the insecurity of its neighbours. The walls were defensive from a tactical point of view but they were aggressive on the strategic level. After the fortification, Athens saw no need to consider Sparta’s concerns or interests.

Nato, nominally a defensive organisation, is the most obvious “Pericles Wall” of our times.

In the name of the “collective security” of its members, Nato threatens the security of other nations in the region by exerting political and strategic pressure on those outside the bloc. Its continuous expansion, even after the end of the Cold War, could be perceived only as a threat by Moscow – particularly after Russia’s proposal to join Nato was rejected out of hand.

The purpose of the United Nations was to substitute the defence treaty system that led to World War I with a mutual security system in which nobody was absolutely secure, but everyone was willing to compromise and negotiate. That idealistic vision was never fully realised. Even so, in the nuclear age, if one wants to avoid the tragedies of the past, we need more bridges and fewer walls.