National unity is one of the components of national power and security which has been always a subject for study by thinkers in the field of political sociology and also has been interested by statesmen in different countries. This issue has attracted more attention in recent decades due to the factors such as fall of colonial system and formation of new nation-states; collapse of the Easter bloc; intensification of ethnic disputes in some countries; globalization; and finally increase in international migrations. One of the subjects which is closely related to national unity is "national identity". National identity is one of the most fundamental elements and a necessary precondition for achieving national unity and solidarity. Since Iran is among the countries that has always, during its long history, been faced with the preservation and strengthening of national unity and solidarity, this article tries to explain relationship between these two concepts in Iran drawing upon a historical sociology approach. To do this, two periods in ancient and modern history of Iran characterized by rise and fall of national unity in Iran were studied. Findings show that in studied periods, there were a direct relationship between national identity and national unity and the strengthening or weakening of national solidarity have been influenced by policies and actions of governments regarding national identity.
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