Wait a minute! Part of the Russian invasion CAME FROM Belarussian territory. I think Ukraine is prudent to establish defenses against a repeat from that sector.
Putin is pressuring Lukashenko to send troops to Poland to stop NATO weapons transfers through the area. Not just Ukraine. That’s why Poland also built a wall on their border with Belarus. The warning to Putin is that if he uses a nuke on Ukraine, he loses Belarus and Kalingrad.