Why did the Persians and Greeks fight in the Persians War?

The endless wars fought between the Greek city-states were spilling over and disrupting the Persian Empire, which had several hundred Greek city-states within it. Persia decided to bring them all nder control, and used their usual tool of bribing the upper class to agree. This worked for many of them, but the rest resisted and Persia invaded to impose tyrants to keep the cities at peace under a Persian provincial governor.