The goal of Russia increased military spending after 1900 was primarily to

stop and oppose Japan

Explanation:

After the year of 1900, Russia started to invest heavily in its military. It may come weird as this the period before any of the two world wars, but the Russians actually had a very big problem with the ever growing power and ambitions of the Japanese. Japan had its eyes on taking over some Russian Islands and important ports, as well as having the goal of invading territories that were of big interest for the Russians, such as Manchuria and Korea. There were conflicts between the two already, so seeing the military advancements in Japan, the Russians feared that they will be defeated in the next one, so rushed to strengthen their military.