20,000 is a division-plus or a small corps. Not really enough to go deeply into the Ukraine even if we assume there won’t be a lot of opposition.
Like the US in Iraq, the Russians might be able to slice & dice in the opening phase, but once they ‘go static’ then the real problems begin.
it seems the US/NATO forces were to have some joint military exercises somewhere in the region. This may just be playing tit for tat.