Did you mean unclear war, or nuclear war?
In conflict there's an escalation process. In the case of Ukraine, for example, NATO and the US are staying out of direct conflict because Ukraine is not a NATO member and it doesn't have a defense agreement with either. Sending troops in Ukraine would be considered an escalation with greater ramifications. You will note that the US has also expressedly requested that Ukraine doesn't shell Russian territory, for the same reason.
However, the US and NATO are openly arming Ukraine to show Russia that it can and will escalate to prevent a takeover of the country and to support Ukraine's efforts to push Russia out of its territory. Russia is refraining from expanding its conquest beyond the land they sought to control.
This is deemed an acceptable status quo, the result of the weeks to come will determine the future of this war. If Russia decides to expand the scope of its conquest, you can be sure that the west will respond accordingly.