According to scholars in this field, the main power of the modern state is the power to be sovereign. This means that, within the borders of the state, the state can do whatever it wants and is the supreme authority.
Perhaps the most important aspect of this sovereignty is the ability to coerce people to do things. Within a state's boundaries, the state is the only entity that has the right to engage in violence and coercion. It has the right to commit violence against those who refuse to obey its laws.
The idea of the sovereign state is very important in international relations, especially for realists. For them, the state is the basic unit of analysis. In addition, the realists' entire view of the international order is based on the idea that states are sovereign and do not have to obey any law higher than their own.