Mark Esper, Trump's Secretary of Defense, said on CNN that Uncle Sam can very much tell when a nuke is rolled out of storage and can implement a lockdown of Ukraine's airspace if that happens.
Regardless of having served under Trump, the man would have been briefed on American capabilities.
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Beyond that, the Ukrainians have morale and discipline. While Russians are told to occupy positions somewhere in the forest and by the look of their abandoned positions simply camp out, Ukrainians still keep establishing proper dugouts.