In the northeastern part of the island, in a location called “Kavos Vasili”, the archaeologists have discovered the ruins of a settlement of the Early Bronze Period. This settlement is the oldest of the wider area of Trizinia, Peloponnese, and is believed to be interrelated with the wreck found on the nearby Dokos island which dates to the same period.
The ancient polis of Kalaureia was home to an asylum dedicated to Poseidon, the ruins of which are still accessible on a hilltop close to the town. This asylum may have been linked to the sanctuaries at Geraistos and Tainaros. Ancient historians claimed that Poros was home to an Amphictyony in the Archaic period, a league of the poleis Poros, Athens, Prassiai, Aegina, Epidaurus, Hermione, Troizen, Nauplion and Orchomenus. An enormous feast was found dating to the Hellenistic period in the ruins of the Kalaureia asylum, along with a plaque celebrating the "revival" of the Kalaureian League.
Poros was divided in two islands during the antiquity: the one was called Sfairia and is the part of the island where today is located the island’s capital; the other was called Kalavria and is the bigger part of Poros at the north of Poros Town. During the Mycenaean dominance (1,400-1,100 B.C.) Kalavria was quite powerful as the most important naval base of the wider region was located on islet Monti or Liontari in the eastern coast of Poros. In the 7th century B.C., it is believed that Kalavria was part of an “amphictyonia”; that is an alliance between multiple City-states. The amphictyonia was named “Amphictionia of Kalavria” and its members were Athens, Poros, Aegina, Epidaurus, Hermione, Trizina, Nafplio, Orchomenos and Prasies. During the 5th century, the Persians started attacking the Greek territories along with the Aegean islands. With the beginning of the Peloponnesian War, which also affected the islands of the Argosaronic Gulf, Trizina and Kalavria offered asylum to the anti-Macedonian politician who eventually became the tyrant of the region. After the death of Alexander the Great in 323 B.C., the Ptolemies of Egypt occupied Kalavria. Around that time, the famous orator Demosthenes came to the island and some say that this is the place where he committed suicide. In 273 B.C., the last explosion of the Methana volcano dramatically changed the morphology of Poros and the wider region.
ROMAN AND BYZANTINE TIMES
During the Roman period (86 BC to 395 AD) Poros was part of the Roman Empire along with Trizina, to which it was a tributary. In Byzantine times, Poros and other islands were often raided by the pirates that dominated the Aegean Sea.
In 1484 the Venetians occupied Poros and used it as a strategic port against their sea battles with the Ottomans. Poros was the most powerful city of the wider area, to which belonged (and also was managed by headquarters in Poros) Methana Island, Epidaurus, Damalas (Trizina), Fanari and Valario. During that time, the island had about 15,000 inhabitants, which means that it was one of the largest cities in Greece. The Venetian dominance ended in 1715.
THE OTTOMAN PERIOD
The Ottoman Period began in 1715, much later in Poros than in the rest of Greece. Shipping and commerce were the inhabitants’ main activities, but Poros’ fleet wasn’t as famous as Hydra’s or Spetses’ fleet, due to the fact that it did not participate in many sea battles.
THE GREEK REVOLUTION
Poros had an important role during the Greek Revolution in 1821, due to its strategic position. The Greek revolutionary leaders, often met in Poros in to discuss and plan their future actions. The first Greek naval base was established in Poros in 1828 and remained there until 1878. In September 1828, the ambassadors of England, France and Russia met in Poros with Ioannis Kapodistrias in order to determine the borders of the future Greek state, which was established two years later, in 1830.
RUSSIAN NAVAL BASE
With the Treaty of Kuchuk Kainarji, Russia secured free shipping for its navy, war and merchant alike, throughout the waters of the Ottoman Empire. As Russian naval activity grew, need arose for a supply station, and land was acquired at the edge of Poros town. Extensive materiel, coal, and food storage facilities were built, as well as a hardtack baking factory. After Greek independence, Governor Capodistrias requisitioned the facilities for use of the Greek war navy, and offered the Russians an alternative location in a nearby cove. The new facilities were far larger, and were used by Russian ships throughout the 19th century. The number of Russian residents of Poros increased and even a Russian school was established. Then as Russian naval activity declined, so did the base and by the early 20th century only a single Russian watchman was left guarding it. It was then granted to the Greek Navy by the Czar but was never put to actual use, and the abandoned buildings were left to decay. The ruins, in elaborately carved stone, were listed as protected architectural monuments in 1989.
In the beginning of the twentieth century, among the activities of the Poros’ inhabitants were agriculture (mainly wheat, grapevines and olives), livestock, fishing and shipping.