The crucial difference, in case is not clear, is that a missile placed in Ukraine would hit Moscow, the capital of Russia, in minutes. This is much more threatening to core Russia interests and Putin would not live with it.
Before rushing to putting all the blame on Russia just because "Putin-bad": I would strongly encourage people who are interested to know more to watch the following talks, from ages ago, given at reputable US universities because let's be honest, none of us is geopolitical historian two weeks ago on this topic so why don't we all do a bit more research.
[1] A good overview of the history, 7 years ago from a talk @ U Chicago. (https://www.youtube.com/watch?v=JrMiSQAGOS4)
[2] Another one from a talk @ Yale: https://www.youtube.com/watch?v=8X7Ng75e5gQ