It is, but he is weakening the credibility of the United States in the process. Never make a threat you aren't willing to back, otherwise everyone knows you make idle threats.
It's just another military adventure ending in a disaster - probably the most humiliating in a long long time. But to your point, it's better for the US to admit defeat now, than in 2 or 3 weeks, let alone in 2 or 3 years. If a parallel can be made, Russia would have been best advised to have done the same 3 years ago.
It only requires reading your refusal to give a straight answer, eg your deflection to a passive-voice observation of the Roman Empire's collapse while avoiding the actual question of how you characterize an explicit threat to terminate a civilization.
