Russian sphere of influence: neighboring countries and parts of former U.S.S.R. and Russian Empire, the Balkans, Eastern Europe, Syria, various vassal states from the past with oil and sea access being major attractions
Russian non-U.N approved military actions in the 21st Century: Chechnya, Georgia, Ukraine (Crimea)
U.S. sphere of influence: the Western hemisphere (see Monroe Doctrine), Asia, Western Europe, the Mediterranean, the Middle East, and really anywhere in the entire freaking world with oil or anything else that we value or find strategic
U.S. non-U.N. approved military actions in the 21st Century: Afghanistan, Iraq, Somalia, Yemen, Pakistan (partial list? who knows for sure?)
What would the U.S. do if political unrest and street protests caused the Cuban regime to teeter and fall? Is it possible that U.S. troops might find their way to Havana and Santiago to make sure that the next group of leaders are more friendly to U.S. interests? After all the U.S. has maintained an embargo against Cuba for over 50 years and has a strategic naval base there (like the Russians in Crimea). The U.S. intervened militarily in Haiti at twice in Haiti in the 20th Century the last time in 1994, so why would Cuba be any different? The U.S. is not likely to annex any territory soon, but neither is the U.S. going to let the "opportunity" of unrest in a coveted territory be wasted without wielding as much power is necessary to achieve a favorable outcome.
I am not trying to justify what Russian has done in Crimea. It is opportunistic and not all that surprising when viewed through the lens of great power politics. It is not radically different from actions taken very recently by the U.S. Might may not make right, but it often does make reality.