You are dead wrong. Ukrainian nationhood goes back longer then America. It’s no more artificial then Poland, which disappeared from the map of Europe for 2 centuries. Both the concept of Italy, and Germany are younger, arising in the 19th century. Prior to that they were collections of individual regions and states.
The Monroe Doctrine stated we would not allow foreign intervention in our hemisphere. It wasn’t a blank check to invade our independent neighbors.
Not the original Doctrine, no. Though it should be noted we didn't have the power to enforce it when issued, and in practice depended on the Royal Navy to do so.
However, in 1904 TR issued an expanded version, called the Roosevelt Corollary, in which he stated that since USA prohibited European powers from enforcing their legitimate claims against American states, USA would enforce those claims for the Europeans. And of course we'd enforce our own claims, too.
In practice this did indeed work out to mean "a blank check to invade our independent neighbors."
See Mexico, Dominican Republic, Haiti, Cuba, Nicaragua, etc. All in the first 30 years of the last century.
Most or all of Ukraine was part of Poland (or Crimean Khanate/Ottoman Empire), then was added section by section to the Russian Empire. Never had an independent existence at all, except during brief and unsuccessful rebellions. This is as opposed to Poland, which had something like 7 centuries of independent existence as a state when it was finally partitioned.
That disappearance, BTW, took place in 1795, and Poland reappeared in 1918. So 123 years, which isn't exactly two centuries.
You are correct, however, about the very concept of nationalism. It didn't take root, at least in central and eastern Europe, till the 19th century. So talking about conflicts between the Russian, Polish and Ukrainian "nations" in earlier centuries is at best anachronistic.
Multiple "republics" existed in Ukraine for varying lengths of time between 1917 and 1919. None were ever stable or organized enough to really be considered independent by any logical standard.