According to the topography in Kyiv, the city is separated by Dnieper river into two parts: north east and south west. Any Russian troop coming from north east Kyiv will be blocked by the wide Dnieper river. In order to get in to the capital city, the Russian troops must cross the river by four bridges. If these bridges are guarded or destroyed by Ukrainian then it will take Russian troops ages to break through!  

However, the invasion of Belarus is a different story because its troops are coming from the south west. The south west side of Kyiv has no river or mountains to slow dow the tanks as it is a flat ground. This situation causes more difficulty for defending in the city. 

The international community put a severe sanction on Russia but they forgot to put the same punishment on Belarus! The Belarus is the most critical threat to Ukrainian defending forces!