The USA has however not done so.
Yes, it has. In Iraq.
As a side note, you should know that the USA does not act as occupiers in any of those countries.
You can take it to the nearest bank that if the US detected that you, Germans, don't behave, your American guests would morph into occpiers in ten minutes.
Which is neither here, nor there as far as Crimean referendum goes. Mere presence is irrelevant if the people were free to vote as they wished. So, unless you have evidence they were not free to vote as they wish, all the crap from you is irrelevant.