The play is getting Russia to take a part of Ukraine for itself in exchange for NATO membership for Ukraine. As a kind of buffer zone for Russia. Obviously, the US would gladly give up whatever part of Ukraine and at least half of the Ukrainian population in exchange for encircling Russia more.
They are gambling that Putin would take territory, since the average Russian would see it as a much bigger win than the usual boring geopolitics manoeuvring. They can put Ukraine into NATO if Ukraine loses and Russia agrees to it, the US would control Ukraine completely if that happened.
The follow up tweet is quite good;