In the year 2049, Siberia has thawed into useable farmland, an opportunity the neighboring Japanese Corporate Republic wishes to capitalize on. So, japan buys a huge chunk of coastal land from Russia. Moscow sees this as an encroachment on its sovereignty, and declares war on Japan on February 4, 2050, a capability the huge nation still has, despite being a member of the Eurasian Union. In addition to this, 4 years prior, the Japanese lunar colony of Tsukuyomi began mining in an area claimed by Korolev, Russia's lunar colony, severly straining diplomatic relations between the two countries.
The first move of the war is a cyber attack by Russia on Japan's infrastructure. For most countries this wouldn't be as detrimental as it was for Japan; almost every aspect of Japanese culture revolved around information technology, from politics to health implants. After this attack, Russia launches several small but powerful missiles at strategic points in Tokyo, hoping to cripple Japan's political infrastructure. However, the unique way that Japan's government is organized causes these attacks to have little effect. Japan is a corporate republic, basically utilizing a system where instead of parties elected into office, megacorporations, robust entities in their own right, put forward one representative each, who represents their respective megacorporation and any potential services that company can offer, leading to a very decentralized, market-run political system. The Russian attacks failed to do anything more than bring down one of many megacorporations. Though this company was the governing entity in power at the time, the next megacorporation in the chain of command filled the vacant positions. This new corporation entering the role of leader was Yutanoshi, Japan's biggest defense contractor. A few Yamato stealth bombers dropping guided bombs on major Russian military assets, and Russia was forced to change strategies. They would cut Japan off from its supply of raw materials:the Tsukuyomi lunar colony.