Funny thing about empires is that the meaning of their existence is to expand. They either expand or collapse. Hence expansion is of existential importance for them. I guess this has to do with the principle of their growth: it's extensive, instead of intensive. https://en.m.wikipedia.org/wiki/Extensive_growth
Another reason why the Russian empire wants Ukraine is to secure its borders by mountain ridges - in this case, Carpathian mountains. Nowadays it makes less sense, since missiles quickly fly over any mountains - but their archaic thinking, induced by former invasions, still persists.
I think we'll have to train a lot of dogs to deal with this problem, and use bacterial biosensors. Ukraine is also developing modern 3-D mine detectors which can do much more than detect just metal.