How did the world differ after the Great War from the world before it? Please consider international relations, international economic systems, culture, or some other theme.
I will look at this issue from the perspective of international relations. There were definitely important differences between the world before this war and the world after it.
One of the major differences was that the world (or at least Europe, which dominated the world at that time) before WWI ran on a system of “balance of power.” What this meant is that there were a relatively large number of major powers, none of whom were in permanent alliances. The powers were relatively evenly matched and therefore no power was able to become a hegemon. Whenever a country did seem to be trying to dominate, two or more other countries would create a temporary alliance to balance out the power of the first country. This prevented any country from becoming strong enough to dominate.
After WWII, this changed to a large degree. Now, there were not as many powers that were evenly matched. Perhaps just as importantly, each country was not willing to ally with each other country. For most of the time between WWI and WWII, the USSR was something of a pariah and other countries in power in Europe would not ally with it. The same thing went for Germany, which was in disgrace after having allegedly been the cause of the war. France and England, for example, were not going to contemplate alliances with Germany.
A second major change was in the way that peace was to be kept. Before the war, peace was kept through the balance of power. After the war, countries believed that peace would be kept through international law and treaties. The creation of the League of Nations was an example of this. So was the creation of the Kellogg-Briand Pact. So were the arms limitations treaties that came out of the Washington Naval Conference. Before the war, there had been no thought that such legalistic and peaceful means could be used to maintain peace.
Thus, the nature of alliances and the means of keeping the peace were very different before and after the Great War. This represented a major change in international relations.