The result was an unprecedented global calamity. The collapse of the British-dominated world order on the oceans, the disruption of the uneasy balance of power on the European continent due to the rise of a powerful unified Germany, combined with the rise of Japanese power in East Asia all contributed to a highly competitive international environment in which dissatisfied great powers took the opportunity to pursue their ambitions in the absence of any power or group of powers to unite in checking them. These were the unsettled, disordered conditions that produced the fertile ground for the two destructive world wars of the first half of the 20th century. If they were to accomplish their aims of establishing hegemony in their desired spheres of influence, the world would return to the condition it was in at the end of the 19th century, with competing great powers clashing over inevitably intersecting and overlapping spheres of interest. However, it is the two great powers, China and Russia, that pose the greatest challenge to the relatively peaceful and prosperous international order created and sustained by the United States. If accomplished, that, too, would have effects on the global order. Finally, at a much lower level of concern, there is the effort by ISIS and other radical Islamist groups to establish a new Islamic caliphate in the Middle East. North Korea seeks control of the Korean peninsula, which if accomplished would affect the stability and security of northeast Asia. At a lesser but still significant level, Iran seeks regional hegemony in the Middle East and Persian Gulf, which if accomplished would have a strategic, economic, and political impact on the international system. China and Russia pose the greatest challenges to the world order because of their relative military, economic, and political power and their evident willingness to use it, which makes them significant players in world politics and, just as important, because the regions where they seek strategic hegemony-Asia and Europe-historically have been critical to global peace and stability. Their aim is to gain hegemony in their respective regions. The external challenges come from the ambition of dissatisfied large and medium-size powers to overturn the existing strategic order dominated by the United States and its allies and partners. The liberal world order established in the aftermath of World War II may be coming to an end, challenged by forces both without and within.