Crimea is an autonomous parliamentary republic that is governed by the Constitution of Crimea which established its autonomy and authority within Ukraine. The Crimean Autonomous Republic has its own independent budget and manages the sum of its territory as its own property.
On May 5, 1992 the Crimean parliament declared itself to be independent; its Constitution became the law of the land on the same day. Ukraine unilaterally annulled this declaration seven days later. Ultimately in 1992 a compromise was agreed to between the Ukrainian and Crimean parliaments and Crimea reverted to being an autonomous republic within Ukraine.
In 1994 the Crimean parliament wrote a new Constitution for itself. In 1995 Ukraine under pro-Russian president Leonid Kuchma began direct rule of Crimea by presidential decree. Thus the Crimean people's desire to see to their own affairs was quashed by totalitarianism from Kiev. Later in 1995 a new Crimean Constitution was drafted and ratified by all parties in December 1998. Article 135 of the Ukrainian constitution mandates that any Crimean constitution must be approved by the Ukrainian parliament.
Herein lies the crux of Vladimir Putin's claim of indigenous Crimean forces (ethnic Russians for the most part) overthrowing totalitarian rule from Kiev. What we know about this is that Russian Spetsnaz (Special Forces) made this overthrow possible. The Russian actions constitutes preemptive foreign military intervention in the affairs of a foreign state and one of its autonomous republics.
In my assessment what this crisis is about is an undemocratic coup d'etat in Ukraine that was followed in very short order by another undemocratic coup d'etat in Crimea which was born of over 20 years of totalitarian struggles between former Soviet Republics.
In my opinion we (NATO/US) do not have a dog in this fight. This is about former Soviet states continuing to sort their own affairs out. All of Ukraine should put its existence to a referendum, let the individual Oblasts and autonomous regions and cities of Ukraine decide who they want to be a part of, or not be a part of, and let them do it peacefully.