Guantanamo today is part of a lease. US pays Cuba money for the land and usage for the base ever year. Cuba doesn't accept payments so it sits in a bank account at treasury. Cuba also has guarantee from the US that US will not invade Cuba if the lease is not extended in the future (agreement during Cuba Missile Crisis). At some point Guantanamo will be back in Cuban hands as the US is actually using Gitmo in violation of agreed terms.
US itself couldn't legally annex Cuba in 1902 because US Congress said no. Russia on the other hand doesn't mind annexing what was once Russian land. I am not saying Russia didn't "invade" or annexed Crimea, I am just telling you this is NO different then what the US has done several times.