[Why interests does the US have in the Black Sea?]
That same interests that we have in the Western Pacific - the defense of our allies against encroachment by a massive foreign power. Russia is already 2x the size of the next largest country on Earth. Do we want it to be 3x, 4x or 5x before we start trying to prevent it from getting any bigger? Germany had a case for lebensraum - it was 1/35 the land area of present-day Russia before it decided to conquer the world. That Russia, at almost 2x the land area of the next largest country (Canada), wants more land is an indication that its land hunger is limitless.