<P> The ongoing conflict in China led to a deepening conflict with the U.S., where public opinion was alarmed by events such as the Nanking Massacre and growing Japanese power . Lengthy talks were held between the U.S. and Japan . When Japan moved into the southern part of French Indochina, President Roosevelt chose to freeze all Japanese assets in the U.S. The intended consequence of this was the halt of oil shipments from the U.S. to Japan, which had supplied 80 percent of Japanese oil imports . The Netherlands and Britain followed suit . With oil reserves that would last only a year and a half during peace time (much less during wartime), this ABCD line left Japan two choices: comply with the U.S. - led demand to pull out of China, or seize the oilfields in the East Indies from the Netherlands . The Japan government deemed it unacceptable to retreat from China . </P> <P> The League of Nations was an international organization founded after World War I to prevent future wars . It failed . The League's methods included disarmament; preventing war through collective security; settling disputes between countries through negotiation diplomacy; and improving global welfare . The diplomatic philosophy behind the League represented a fundamental shift in thought from the preceding century . The old philosophy of "concert of nations", growing out of the Congress of Vienna (1815), saw Europe as a shifting map of alliances among nation - states, creating a balance of power maintained by strong armies and secret agreements . Under the new philosophy, the League was a government of governments, with the role of settling disputes between individual nations in an open and legalist forum . The United States never joined, which lessened the power and credibility of the League--the addition of a burgeoning industrial and military world power might have added more force behind the League's demands and requests . </P> <P> The League lacked an armed force of its own and so depended on the members to enforce its resolutions, uphold economic sanctions that the League ordered, or provide an army when needed for the League to use . However, they were often very reluctant to do so . After numerous notable successes and some early failures in the 1920s, the League ultimately proved incapable of preventing aggression by the Axis powers in the 1930s . The reliance upon unanimous decisions, the lack of an armed force, and the continued self - interest of its leading members meant that this failure was arguably inevitable . </P> <P> In the late 1980s the British historian Richard Overy was involved in a historical dispute with Timothy Mason that mostly played out over the pages of the Past and Present journal over the reasons for the outbreak of World War II in 1939 . Mason had contended that a "flight into war" had been imposed on Adolf Hitler by a structural economic crisis, which confronted Hitler with the choice of making difficult economic decisions or aggression . Overy argued against Mason's thesis, maintaining that though Germany was faced with economic problems in 1939, the extent of these problems cannot explain aggression against Poland and the reasons for the outbreak of war were due to the choices made by the Nazi leadership . </P>

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