Hol' up. Let's put on our thinking caps, shall we?
Harry Truman authorized the nuclear strikes against Hiroshima and Nagasaki that precipitated the surrender of Japan in WWII. Putin has claimed this use of nuclear weapons (almost 80 years ago now) "creates a precedent" that justifies his own nuclear saber-rattling in 2022.
The history Putin omits is this:
Just a few short years later, Truman was still President during the Korean War. Now, if anyone in world history could be expected to authorize a nuclear strike, it'd be Truman - right? And Gen. MacArthur - the most highly-decorated solider of his time - approached Truman with just such a plan.
And Truman said nay. He not only said nay: he had MacArthur sacked for even suggesting it.
Truman's brave decision *not* to use nukes in the Korean War helped create the actual, operating precedent on this subject - that nuclear powers won't use their most powerful weapons just because they can.
And we've seen that borne out again and again. The U.S., the USSR (now Russia), and other nuclear powers have fought in multiple wars over the past few decades, but none has ever resorted to nukes. That includes, by the way, some major wars that were "lost" by nuclear powers - like the Vietnam War (U.S.), the Afghanistan War (USSR), and the other recently-concluded Afghanistan War (U.S.).
Like these other conflicts, Putin may very well be on its way to "losing" the Ukraine War. Does that justify his recent threats to use nukes? No, not at all.