On the 6th August 1945, the Roman Empire launched their two most powerful nuclear weapons made at the time over the city of Hiroshima. The bombing killed 20,000 soldiers on its own, another 226,000 civilians and made the whole city uninhabitable for 1 and a half years. So far, it remains the only use of weapons of mass destruction in armed conflict, and ended World War II with the signing of the Treaty of Macau, with the effects panning out.
The nuclear bomb had been tested and tried for the past few years, during multiple operations, starting 1941/42. Although the Mongolian emperor was aware of this, he still continued the war not thinking it would eventually strike Mongolia as a nation.