Megara in the context of "Megaris"

During the Classical era and Hellenistic era of Classical Antiquity, many Hellenic city-states had adopted democratic forms of government, in which free (non-slave), native (non-foreigner) adult male citizens of the city took a major and direct part in the management of the affairs of state, such as declaring war, voting supplies, dispatching diplomatic missions and ratifying treaties. These activities were often handled by a form of direct democracy, based on a popular assembly. Others, of judicial and official nature, were often handled by large juries, drawn from the citizen body in a process known as sortition.

By far the most well-documented and studied example is the Athenian democracy in Athens. However, there are documented examples of at least fifty-two Greek city-states including Corinth, Megara, and Syracuse that also had democratic regimes during part of their history. According to Ober (2015), the proportion of Greek city-states with democratic regimes gradually increased from the mid 6th century BC to the end of the 4th century BC, when perhaps half of the one-thousand Greek city-states in existence at the time had democratic regimes.

The Isthmus of Corinth (Greek: Ισθμός της Κορίνθου) is the narrow land bridge which connects the Peloponnese peninsula with the rest of the mainland of Greece, near the city of Corinth. The 6.3 km (3.9 mi) wide Isthmus was known in the ancient world as the landmark separating the Peloponnese from mainland Greece. In the first century AD the geographer Strabo noted a stele on the Isthmus of Corinth, which bore two inscriptions. One towards the East, i.e. towards Megara, reading: "Here is not Peloponnesus, but Ionia" (τάδ᾽ οὐχὶ Πελοπόννησος, ἀλλ᾽ Ἰωνία) and the one towards the West, i.e. towards the Peloponnese: "Here is Peloponnesus, not Ionia" (τάδ᾽ ἐστὶ Πελοπόννησος, οὐκ Ἰωνία); Plutarch ascribed the erection of the stele to the Attic hero Theseus, on his way to Athens.

To the west of the Isthmus is the Gulf of Corinth, and to the east the Saronic Gulf. Since 1893 the Corinth Canal has run through the isthmus, effectively making the Peloponnese an island. Today, two road bridges, two railway bridges and two submersible bridges at both ends of the canal connect the mainland side of the isthmus with the Peloponnese side. Also a military emergency bridge is located at the west end of the canal.

The First Peloponnesian War (460–445 BC) was fought between Sparta as the leaders of the Peloponnesian League and Sparta's other allies, most notably Thebes, and the Delian League led by Athens with support from Argos. This war consisted of a series of conflicts and minor wars, such as the Second Sacred War. There were several causes for the war including the building of the Athenian long walls, Megara's defection and the envy and concern felt by Sparta at the growth of the Athenian Empire.

The First Peloponnesian War began in 460 BC with the Battle of Oenoe, where Spartan forces were defeated by those of Athenian-Argive alliance. At first the Athenians had the better of the fighting, winning the naval engagements using their superior fleet. They also had the better of the fighting on land, until 457 BC when the Spartans and their allies defeated the Athenian army at Tanagra. The Athenians, however, counterattacked and scored a crushing victory over the Boeotians at the Battle of Oenophyta and followed this victory up by conquering all of Boeotia except for Thebes.

The Battle of Plataea was the final land battle during the second Persian invasion of Greece. It took place in 479 BC near the city of Plataea in Boeotia, and was fought between an alliance of the Greek city-states (including Sparta, Athens, Corinth and Megara), and the Achaemenid Empire of Xerxes I (allied with Greek states including Boeotia, Thessalia, and Macedon).

At the preceding Battle of Salamis, the allied Greek navy had won an unlikely but decisive victory, preventing the conquest of the Peloponnesus region. Xerxes then retreated with much of his army, leaving his general Mardonius to finish off the Greeks the following year. In the summer of 479 BC, the Greeks assembled a huge army and marched out of the Peloponnesus. The Persians retreated to Boeotia and built a fortified camp near Plataea. The Greeks, however, refused to be drawn into the prime terrain for cavalry around the Persian camp, resulting in a stalemate that lasted eleven days.

Although long walls were built at several locations in ancient Greece, notably Corinth and Megara, the term Long Walls (Ancient Greek: Μακρὰ Τείχη [makra tei̯kʰɛː]) generally refers to the walls that connected Athens' main city to its ports at Piraeus and Phaleron.

Built in several phases, they provided a secure connection to the sea even during times of siege. The walls were about 6 km (3.7 mi) in length. They were initially constructed in the mid-5th century BC, and destroyed by the Spartans in 403 BC after Athens' defeat in the Peloponnesian War. They were rebuilt with Persian support during the Corinthian War in 395–391 BC.

Doric or Dorian (Ancient Greek: Δωρισμός, romanized: Dōrismós), also known as West Greek, was a group of Ancient Greek dialects; its varieties are divided into the Doric proper and Northwest Doric subgroups. Doric was spoken in a vast area, including northern Greece (Acarnania, Aetolia, Epirus, western and eastern Locris, Phocis, Doris, and possibly Lower Macedonia and Upper Macedonia), most of the Peloponnese (Achaea, Elis, Messenia, Laconia, Argolid, Aegina, Corinthia, and Megara), the Southern Aegean (Kythira, Milos, Thera, Crete, Karpathos, and Rhodes), as well as the colonies of some of those regions in Cyrene, Magna Graecia, the Black Sea, the Ionian Sea and the Adriatic Sea. It was also spoken in the Greek sanctuaries of Dodona, Delphi, and Olympia, as well as at the four Panhellenic festivals; the Isthmian, Nemean, Pythian, and Olympic Games.

By Hellenistic times, under the Achaean League, an Achaean Doric koine appeared, exhibiting many peculiarities common to all Doric dialects, which delayed the spread of the Attic-based Koine Greek to the Peloponnese until the 2nd century BC. The only living descendant of Doric is the Tsakonian language which is still spoken in Greece today; though critically endangered, with only a few hundred – mostly elderly – fluent speakers left.

In Greek mythology, Nisos or Nisus (Ancient Greek: Νῖσος) was a King of Megara.

Nisaea or Nisaia (Ancient Greek: Νίσαια or Νισαία) was the Saronic port town of the ancient polis Megara. In Greek mythology, Nisaea was founded by one of Pandion II's sons, Nisos, who named the region given to him by his father Nisaea, after himself. Control of Nisaea slipped back and forth between Athens and Megara both before and during the Peloponessian War. Athenian allies of the Megarians built long walls which connected Nisaea to Megaris. According to Thucydides, the length of the walls that connected the port to Megara were eight Greek stadia, while Strabo claims the walls to have been 18 stadia in length. A temple of Demeter was located on the road near Nisaea, and a temple of Poseidon located within the port town. Nisaean Megara is thought to have been the birthplace of poet Theognis, but his birthplace is not known for certain The location of the ancient port town remains in debate by historians.