Besides the theories of IR one must grasp we basic concepts that underpin contemporary world order such as globalization, nation-states in the contemporary world, and nationalism.
There is no way to cover all these from a single source, in my view, IR is a demanding and technical subject. After going through multifarious sources, I found the book written by jhon bayalis and steve smith is a remarkable piece of work to grasp above mention concept if one find hard to understand these concepts and their implication on contemporary world order one must give a read to this book
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