It’s a kind of paradox: the concept of international system which seems to be so obvious and so important has not been used in international relations theory. That’s particularly strange as the realist theory gives us a macro vision of international politics, but realism was a little bit reluctant to the concept of system which implies an idea of order. For the realists, there is no order in the international arena as it is a permanent fight among gladiators, it’s a kind of anarchy. And so, we had to wait for the great moment of the system theory for observing the first uses of the concept of the international system, that’s to say around the 60s, the 70s, so, very late. My purpose is to define what is an international system, which is not so obvious too. How to define it? And after, I will offer a lecture on what is multilateralism, that’s to say how multilateralism is shaping the international system. And then we have to take into account the famous problem of polarity. Are we facing a unipolar or a multipolar or a post-bipolar system? What does it mean? What is an international system? I would say that an international system is a state of international life, a moment, a sequence of the international life. It is made for helping to describe and to understand constraints on actors and relations. It is I would say a set of international practices that can be identified at a given sequence of the time. Now the problem is to define the criteria of the international system. If the international system is a moment of the international life, how to seize this moment? How to describe this moment? Which kind of variables has to be used and mobilized for describing the international order? I would select four criteria, four variables, which are explaining the transformation of the international system. First one is degree of inclusiveness. Is the international system including all the potential actors? All the potential actors, it means first all the states, all the sovereign states. And we have to take into account something very important and even crucial for understanding the international system that generally all the state actors don’t participate in the international system. If we observe the European 19th century international system, it was limited to Europe. It pretends to cover all the world and to take into account what for instance took place in Latin America, but however only European states were participating in it. The China or the Ottoman Empire were not admitted as members of the international system, and with the Monroe doctrine, US didn’t accept to participate in this kind of international system. And now the problem is what about developing countries? Formally of course they are members of the international system, but sometimes, late members. If you take into account for instance the decolonization process, this decolonization process took place during the 60s, mainly during the 60s, and so, before 1960 the major part of the world was not actively participating in the international system. Now, another question is: what about the non-state actors? That’s to say: is the inclusiveness of the international system also covering non-state actors, which are more and more active, more and more relevant inside the international arena, but which don’t really participate in. The second variable would be what about the deliberation? An international system can be identified through its way of deliberating, that’s to say how deliberation is going on? What are the institutions, the procedures? And what are the members of the international system, which are really participating, actively participating in the decisions? And so we have to observe that for the major part of the international system, those one were oligarchic or even diarchic, that’s to say we have to take into account the difference between active states, which are really participated in the decision-making process and passive states. For instance, now, with the P5, that’s to say the five permanent members of the security council of the UN, only five sovereign states are really active states in the international arena, and the others have to follow. If now we take into account the G groups, that’s to say G7, G8 or G20, all the other members are not participating in the decision-making process, and so we can say that this deliberation is really oligarchic, and so we can classify the international systems according to the real participation of the actors to the decision. Third criterion, it would be the alliances which are composing the international system. Are these alliances structured or not? Are they covering all the states of the world or not, or only a small part? Are they lasting like NATO for instance created in 1949 up to now, or are they precarious? This is another variable. And the last one will be of course the type of power and domination, which is exercised inside the international system. Is there a hegemonic power? Is there a condominium? Or is there a decentralized power inside the international system? If now we try to classify the international systems, we have to take into account not to fall into three traps. The first one would be the obsession with polarization considering that all the international systems are polarized, that is not true. Polarization, as we will see, is an exception. What about the non-polarized international systems? The second trap will be the absence of a social dimension, that’s to say international system is not only made of state actors, is not only made of sovereign states, but more and more as we saw previously, more and more by non-state actors. What are these non-state actors? How to classify them? And how to consider their participation in the international life? And the third trap would be disregarding the temporal factor, that’s to say international system is not a stable one, an international system is moving, is transforming, and this transformation is probably at the core of the international analysis. If, for putting an end to this lecture, I consider different kinds of international systems, I would say that for instance beginning with the Vienna congress in 1815, from 1815 up to 1818, we had a kind of oligarchic joined management, that’s to say the fourth winners of the wars against Napoleon were co-managing the international system. From 1818 up to 1871, that’s to say the creation of Germany, I would say that we had an unstable and a conniving coalition, fragile coalition among the major European powers, but without a real command management, a kind of limited competition. From 1871 up to 1914, that’s to say the beginning of the First World War, we had a conniving antagonism, that’s to say structuring alliances, which will result in the confrontation of the First World War. From 1918 up to 1939, that’s to say the Second World War, we had a very fluid international system, without any structuration and without a very fast transformation of the coalitions and alliances. Now from 1945 up to 47, very brief period just after the war, we had a kind of fragile joined management, a kind of condominium between USSR and US who were the two winners of the Second World War, I mean the main two winners of the Second World War. From 1947 up to 67, we had a rigid bipolarity. From 67 up to 89, that’s to say the destruction of the Berlin wall, we had a diarchic polarization From 89 up to 94 we had a limited unipolarity around US which was the real winner of the cold war. And from 94 up to now, we have a kind of fragmented apolarity, without a clear hegemony, without a clear structuration, and because the system is not very clear, it’s coined post-bipolar system, without deciding exactly on the content of this international system.