Explain why the global order twice collapsed into world war in the first half of 20th century?
The World Order collapsed during World War One because of imperialistic designs of major European countries and the burgeoning growth of nationalism, especially among ethnic groups with no designated homeland. It collapsed again a few years after the Versailles Peace Conference because of the utter failure of that conference to establish a lasting peace.
Shortly before World War One, there was a "race" for territories in Africa and Asia as well as some territorial expansion in Europe, particularly in the Balkans. The diplomatic pressures caused by Imperialism were relieved somewhat by the Berlin Conference which resolved issues of African colonization; but still remained. Largely because of these territorial claims and the need to protect each countries interests, a series of military alliances developed. Sadly, they did not make provisions for minorities seeking national identity. It is no coincidence that the war began with an assassination by a young Serbian man who like many others, yearned for independence for Serbia.
At the end of the war, a number of victorious powers, primarily the French, were determined to punish Germany for he war and also to cripple it to prevent a re-occurence. As a result, the terms of the Versailles Peace Conference were excessively harsh and caused tremendous resentment among the German people. This provided an ample theater from which Adolf Hitler could spread his ideas of the unjustness of the Treaty and the greatness of the German people who had been so wrongfully abused (in his opinion.) Thus the entire world order managed to break down twice in forty years.