This chapter uses empirical analysis of over ten thousand of primary and secondary sources to examine the war in Ukraine since the Russian invasion on February 24, 2022. The study analyzes the nature of this most significant armed conflict of the twenty-first century, the involvement of Russia, Ukraine, Donbas separatists, and the West in the war, and the estimates of military casualties. The analysis shows that this war is primarily an interstate war between Russia and Ukraine and a proxy war between NATO and Russia with elements of continuation of a civil war. The Russian invasion of Ukraine was illegal under the international law, contrary to the Russian government claims.