Additionally, the international system is much more complex than a single cause and effect which Waltz claims. It is wrong to assume states are always free to act without any constraints.  Exchanges within an interdependent system, such as the global economy, are ‘mixed motive games’.  Structural realists have failed to take into account the rise of the European Union and underestimated the dynamics of European integration and governance upon state behaviour.  For example, the recent use of the veto power by Russia and China regarding the proposed UN intervention in Syria which America was keen to push through, could be viewed as just one of the constraints upon state behaviour. Thus, it is clear that structural realism is unable to adequately analyse the politics and economics of one of the key areas of world politics. As Helen Milner argues, the effect of interdependence upon states’ behaviour needs to be given at least equal consideration to anarchy. 
Hence, power becomes the second essential key for Realism theory in the international field. States will act according their own interests as "statesmen think and act in terms of interest defined as power". (Morgenthau, 1985a:5) Therefore, states leaded by statesmen pursue power. Hans will show in his work three ways for the struggle of power, but we will study only two: the maintenance of power and the increment of this. This importance of power will be essential to explain later the Spanish foreign policy during World War II.
1 International Relations (IR) (occasionally referred to as International Studies (IS))  is the study of relationships between countries, including the roles ofstates, inter-governmental organizations (IGOs), international nongovernmental organizations (INGOs), non-governmental organizations (NGOs) and multinational corporations (MNCs). It is both an academic and public policy field, and can be either positive or normative as it both seeks to analyze as well as formulate theforeign policy of particular states. It is often considered a branch of political science (especially after 1988 UNESCO nomenclature), but an important sector ofacademia prefer to treat it as an interdisciplinary field of study. of