Russia has been moving troops into position without firing any shots.
From Lonely Planet
Russians started out with 13,000 military personnel in Sevastopol, including 3,000 naval infantry (their marines). I assume some Russian Spetsnaz are in the garrison since the Black Sea Fleet has some.
Reports are that 6,000 new troops entered Crimea since the crisis began. Some were flown in. Others apparently crossed the Kerch Strait from Russia.
So that means 9,000 combat troops plus 10,000 others (probably mostly non-army), some of which could be armed and sent out to defend Sevastopol's perimeter or other static positions along the roads north and east and airports, but which aren't skilled in maneuver warfare.
Russia started with Sevastopol in the southwest.
The Russians are reported to have taken the airport near Sevastopol outside the base; taken Simferopol air base, which lies at a major intersection leading north and east from Sevastopol; taken the narrow Isthmus of Perekop in the northwest where Crimea ends, where they are digging trenches or anti-tank ditches at Armyansk; taken Kerch which secures the entry via ferries from southern Russia; captured an air defense headquarters in Crimea; and are besieging a Ukrainian marine base at Feodisiya on the road from Kerch to Simferopol.
I wondered what was happening at Dzhankoy, which is on a major road from Simferopol leading northeast to Ukraine across some very narrow routes to Ukraine in that direction and also to the major isthmus to the northeast. Sure enough, the Russians took that, too:
Ukrainian media also reported Saturday that Russian troops took over the airport of the Crimean city of Dzhankoi.
So Russia now holds the entry points in the east and north for access to Crimea. Crimea is now cut off from Ukrainian forces and open for further Russian reinforcements flying in to any of the airports captured, by ferry at the Kerch Strait, or by sea directly to Sevastopol port.
I doubt that Russian forces are doing more than securing these lines of communications, so speaking of Crimea as captured is premature.
But the backbone of that eventual control is established.
UPDATE: I suspect that US announcement that Russia has "complete operational control" of Crimea refers to this dominance of the main transportation and communications centers:
A senior U.S. administration official said late Sunday that Russia now has complete operational control of the Crimea peninsula, with 6,000 air and naval forces in the region.
I don't know what that 6,000 figure refers to. It may be garbled in the article.
UPDATE: Per CNN, the figure must mean army and naval infantry. I thought the 3,000 naval infantry started on the ground. Did Russia really only need 3,000 more moved in from Kerch and airlifted in?