Russia has the world's largest arsenal of nuclear weapons. But from Moscow's perspective, the atomic deterrent was not enough to keep the US and its NATO allies from backing Ukraine against Russia.

That was unexpected since the Kremlin views this as a Western intervention in a proxy war that is strategically vital to Russia, Dmitri Trenin, an ex-Russian intelligence colonel and former director of the Carnegie Moscow Center, tells Ian Bremmer on GZERO World.