The possibility of a nuclear war between the United States and Russia cannot be ruled out in the event of a military conflict between the two nations. Both countries possess large arsenals of nuclear weapons, and any military action could potentially escalate into a catastrophic nuclear exchange.

It's important to note that the use of nuclear weapons is governed by a doctrine of mutually assured destruction (MAD), which means that both sides understand the devastating consequences of using these weapons and, therefore, would not use them unless it's a matter of national survival.

However, in the heat of a conflict, a decision to use nuclear weapons may not always be made rationally. Moreover, the possibility of an accidental launch or miscalculation cannot be completely ruled out.

Therefore, it's crucial for both the United States and Russia to maintain open lines of communication and work towards resolving any disputes diplomatically to prevent the possibility of a catastrophic nuclear war.