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A state’s sovereignty is dependent on other states recognizing the state as a state, mainly including the right to determine the religion of the state, the inception of international law, the beginning of embassies, alliances, etc, and more predictability/reliance in international relations.
This idea originates from classical Rome and Medieval Europe. The Treaty of Westphalia (1648) can be seen as the point where the ideas of statehood were formally established.
The principle of sovereignty has been supported by putting ideas such as diplomacy and non-intervention into practice.
Groups such as the Islamic State may call themselves a state, but if **no other states or international organizations recognize them**, they cannot be considered a sovereign state. International legitimacy is essential in order for a state to be considered an independent sovereign country.