The continued existence of a free and democratic Europe could remind US citizens of what they once had and how much better they could be living, which makes the continued existence and success (fingers crossed) of the EU an existential threat to US conservatives.

This is the main way how Ukraine has been a threat for the Russia at least since 2008. Back then, about 40 % of Ukrainins spoke Russian as their mother tongue, and they were suddenly living a comfortable life in a democracy. Putin lives off the Russian assumption that Russians cannot live in a democracy – that the "Russian soul" wants a strong leader so strongly that any attempt at democracy would be futile. And at the same time, he's saying that all people whose mother tongue is Russian "are Russians". So, when people who – according to his propaganda – are Russians, are able to lead a sensible and civilized life in a democratic country, it makes a lot of people who are actually Russians think that their country could maybe achieve the same as well.

And yeah, the above is the most important reason why the Russia betrayed the nation that had come as close to considering the Russia a friend as is in any manner possible. It paints a grim future for the USA-EU relationship.