America (under Trump, who also wasn't a pussy like Obama in that he started slowly arming Ukraine) already smashed Russian military forces (back then still pretending to be a "private military company" but they stopped pretending since, they're the GRU component answerable almost directly to Putin through only his fellow "former" gangster Prigozhin while bypassing the Stavka) in the very same Syria and there was no nuclear response (nor any other). Because they respect strength and fear the strong (to quote Putin's famous saying, "the weak get beaten", he thought Biden would be weak like Obama).
Ukraine is "invading the Russian soil" (post-annexation) and still there's no nuclear response (and probably won't be, people have feared it since the beginning, but making totally clear we're going to let it happen will ensure it in fact won't happen).
America (under Trump, who also wasn't a pussy like Obama in that he started slowly arming Ukraine) already smashed Russian military forces (back then still pretending to be a "private military company" but they stopped pretending since, they're the GRU component answerable almost directly to Putin through only his fellow "former" gangster Prigozhin while bypassing the Stavka) in the very same Syria and there was no nuclear response (nor any other). Because they respect strength and fear the strong (to quote Putin's famous saying, "the weak get beaten", he thought Biden would be weak like Obama).
Ukraine is "invading the Russian soil" (post-annexation) and still there's no nuclear response (and probably won't be, people have feared it since the beginning, but making totally clear we're going to let it happen will ensure it in fact won't happen).