The Chief Justice writes the court's opinion on the case if he or she is a part of the majority. If the Chief Justice does not want to write the opinion, then they choose who from the majority will write it. This was historically common especially since they serve for a long time period.
Japan had few natural resources, including oil, so the country wanted to expand to get access to oil and other resources. As the map of China shows, China had oil and other natural resources.
