Ukraine is not a Baltic state, the Baltic states (Lithuania, Latvia and Estonia) are all in the EU and NATO members. No way the EU would let that slide.
"No way the EU would let that slide." They EU has no ability in military things if the UK is out. The Nato would not let it slide but they also would not fight back to win it back. It would be cold war II.
The entire thing reminds of this[1] Futurama bit. "You won't be needing Harriers anymore, will you? I take Harriers now, F35 come next decade".