As this article points out, the reason has materialized now. The US has started a proxy war against Russia. The fact that would happen has been more or less evident ever since the end of WW2.
For decades, the threat of nuclear armageddon has kept Russia and the West out of a direct confrontation. The prospect of global nuclear war has been a line that neither side is willing to cross.
Opinions? Will Russia ever accept defeat before using its weapons that were accumulated for the purpose now at hand?