One mental exercise that I enjoy is speculating about alternate historic outcomes.
For example, what might have been the course of World War II if the Japanese had attacked the U.S.S.R. (Vladivostok) in the summer of 1941, instead of Pearl Harbour, Hong Kong, and Singapore (December, 1941)? Did the Japanese consider this option?
Would the United States have entered the war without the rationale of a direct attack? (If the entry of the U.S. was delayed, what might have been the impact of the delay?)
The Soviet Union would have been exposed to a two-front war (as Germany was) and — given American antipathy to the Soviet Union — might not have benefited from the American Lend-Lease program. How far might the Japanese have been able to penetrate the eastern regions of the Soviet Union? Would they have gained control of significant resources and/or manufacturing centres? Would the German Operation Barbarosa have ground to a halt if the U.S.S.R. had been forced to re-deploy resources to an eastern front? Would the Battle of Moscow, the Battle of Stalingrad or the Battle of Leningrad have had different outcomes?
If the Japanese had not invaded Hong Kong, and Singapore at the same time, would the U.K. have declared war on Japan?
Would a 90º (one quadrant) war, fought only in east Asia, have focused and conserved manpower and other resources for Japan better than the 360º (four quadrant) war they fought?
Would the United States have undertaken the Manhattan Project with such a sense of urgency if the country was not in the war, and would the Allies or the Germans have been the first to test (and use?) an atomic bomb?
The archives of all nations and of notable leaders will include records of alternate scenarios, contingency plans, etc. If there are any readers who are historians, I would appreciate any historical information you can bring to bear on this speculation. I would welcome any speculations of your own.