Cities are really hard to capture and hold, with a defender advantage of 1:5(or up to 1:10 of defenders to aggressors. depending on who you believe) napkin math

Cities are really hard to capture and hold, with a defender advantage of 1:5(or up to 1:10 of defenders to aggressors. depending on who you believe) napkin maths suggests that a quick city siege is 4 months. So that blows your time budget out of the water. Kiev is reasonably close to resupply (assuming the logistics is managed properly.....) Its far wiser to avoid the cities with a bargepole.