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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might be agreeing to participate ethically with other european nations in the slave trade

or notifying other european nations when taking possession   of african territories

lend lease program was passed by congress on march 11, 1941. the president could ship weapons, food, or stuff to any country whose struggle against the axis assisted u.s. defense