What happened during the 1920s and 1930s to change the direction of both political and foreign policy in Japan? Were the changes that major, or did factors that had been there since the 1870s simply become more prominent?
I'd say that the factors that had been there just got more prominent. After all, Japan had been trying to become more of a power in its region for quite some time before the 1920s. The first Sino-Japanese War and the Russo-Japanese War are just two examples of how Japan was becoming more militaristic and aggressive even before the '20s.