When a conventional war risks nuclear escalation

Can nuclear powers fight a long war without resorting to nuclear weapons?

Possibly, but restraint becomes precarious once either side believes it is losing and has no conventional path to victory.

The answer treats theories of stable conventional conflict as untested because human decisions under defeat are unpredictable. Restraint is most plausible while both parties see an acceptable path to victory. Once one side believes it cannot win, nuclear use and abandonment of other wartime limits become much harder to rule out.