If Crimea’s reunification with Russia hadn’t taken place, the US would have turned it into a protectorate. It would have deployed weapon systems in the peninsula as they are doing in Romania, and thus be able to deny access to Russian vessels to the Black Sea. Ukraine would have given the Sevastopol base to NATO for military purposes.
The narrative is part of the set of arguments to defend Russian annexation of Crimea. The statement actually comes from the head of the Russian Federation Council’s Committee for Defense and Security talking about the deployment of TU-22M3 strategic bombers to Crimea, allegedly as a response to the deployment of US missile defense systems in Romania. Also, Russian senator Alexei Pushkov tweeted that "the first thing Kiev would have done had Crimea not joined Russia would have been turn Sevastopol to the US or NATO".
There is no evidence that the US were planning to deploy any weapons system in Crimea, which was part of sovereign Ukraine. Unlike Romania, which is a NATO country, Ukraine is only part of the NATO Membership Action Plan (MAP), which carries much less obligations and benefits from the Alliance.
In Romania the missile deployment took place according to the European component of the US Ballistic Missile Defense System (EPAA / European Phased Adaptive Approach). It was decided in the NATO Summit in Bucharest in 2008 and an agreement was signed in 2011 between the US and Romanian governments in 2011 regarding to it. Nothing similar has ever emerged regarding to Ukraine.
Further debunking on The Insider.