Nebs Sez
In the military Doctrine of Force Concentration, it is stated that an attacker needs a Force Ratio of three to one over the defender. Ukraine's armed forces consists of 361,000 troops, so the Russians need over a million military personnel in order to secure its outcome. So far it seems the have only committed to 190,000. This is primary reason why they're being checked everywhere. Secondary considerations are the morale of the respective forces and Russia's so-so logistics.