Nationalism is a form of tribalism
Tribalism: the behaviour and attitudes that stem from strong loyalty to one's own tribe or social group
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Tribalism has existed for millennia and can be traced back through history as existing before what we would consider a nation state or even a proto-nation state.
A nation can be defined as a body of people united by common descent and culture, often inhabiting one specific area. Loyalty to this body or group is often seen as inherent: people like to be with others who have the same religion, language and cultural practices, leading them to seek to live together, thus forming the nation state out of nationalism. In prehistoric societies this led to conflicts between different nations who believed that their practices were superior, indicating that nation states were formed to resolve these conflicts so people could live within their own national group. Additionally, the Stanford Encyclopedia of Philosophy suggests that modern nationalism is particularly prominent amongst groups who do not yet have their own state, such as Palestinians and Kurds, indicating that nationalism must predate the formation of a nation state because otherwise, nation states would not develop. 
What creates these loyalties? They are often a result of similar groups living together in proto-nation states like Greek city states, so nationalism is developed by this proximity, which is increased when formal nation states are developed. Without this proximity, tribal groups would not feel the same loyalties to each other, suggesting that states and geographical closeness create the conditions for nationalism.
Tribalism is a prehistoric phenomenon which predates anything resembling a modern nation state, providing strong evidence, with a significant precedent, that nationalism existed before nation states, which are formed as a result of nationalist feeling.
Loyalty to your own group or tribe feels inherent, and is not created by nation states. Nations can be formed as a result of these groups. Many groups which feel a strong nationalism, such as Palestinians and Kurds, do not have a state, meaning nationalism must come first.