Many of the Greek colonies are started in Anatolia. These colonies are along the coast. The Greeks need access to the sea to connect the colonies to their mother polis. Only ships can carry the vast amounts of exports the mother poleis need.
The problem is that Anatolia becomes part of the Persian Empire. The Persians are tolerant for their time. They allow conquered people to keep their language, traditions, and religion. But they have to be governed by a Persian governor in the Persian way of government. This works well with all the peoples of the Persian Empire except the Greeks.
The independent minded Greeks don't like government run by a king. The Persians assign tyrants to rule the Greeks. The Greeks are not to keen on Persians. They consider them just one more group of barbarians. Barbarians do not rank high with Greeks. The Greeks are not cooperative with their Persian overlords.
As time goes on, the Greeks begin to resist their Persian governors. They even stage revolts. The Persian patience is worn out. The Greeks are a pest. They might encourage other peoples in the Empire to rebel.
The Persian use their powerful army to put down the Ionian Revolt. The Greeks in Greece help their Ionian colonists resist the Persians. This prolongs the war the Persian fight to reconquer the Anatolian Greek colonies. It angers the Persian King. He promises himself and his dynasty he will rid himself of meddling Greeks.
The Persian Empire and the people who were in it.
The Persian Empire moves to block Greek access to the Black Sea. This is a major threat to the Greeks. They depend on exports of grain from the Greek Black Sea colonies. Without these exports, Greece will fall far short of the amount of food it needs. The Persians hope cutting off these food supplies will bring the pesky Greeks in line.
Threatening Greek food supplies is like whacking a bee hive with a stick. It angers the Greeks and sends them swarming to protect their sea lanes. And the Persians are at their weakest when it come to fighting at sea.
The Greeks provoke the Persians. In turn, the Persians provoke the Greeks. This can only mean one thing. The Greco-Persian Wars are on.