What brought sparta and athens together as allies?

These city-states were independent of each other, and usually fought wars. However, there are times when city states (such as arch enemies Sparta and Athens) work together, and it is usually forcefully brought to them through outside invasions, such as, for example: Persia, where the invasion from the east brought the two city-states (& others) to fight off the outside influence. Generally after the invasion ended they went back to fighting each other.

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explanation:

the correct answers are 1, 2 5, 6