There was serious discussion in Washington of nuking China after China's entry into the Korean war. (Specifically, nuking concentrations of rice fields, causing starvation until the Beijing relents, at which time Washington would provide food aid.)

When in the 1960s tensions between Moscow and Peking rose, Moscow asked Washington if Washington would be okay with Moscow's nuking China. (Washington replied with a firm no.)

Although Russia is probably not going to resort to nukes, the risk is high enough that I wish my country (the US) had started drastically increasing American preparedness for nuclear war when it started sending military aid to Ukraine.