The correct word in the blank is enmity.
Enmity was incited by the expelled brother, Polyneices, against his brother's leadership of Thebes.
Enmity fuels hostility. The army that Polyneices raised must be hostile against his brother's leadership before it could march against the city. The purpose of fueling enmity against the city was to improve Polyneices' claim to the kingship of Thebes.
Thus, Polyneices wanted to reassert his right to the throne of his father as the elder son.
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