It is important to remember that Ukraine announced a "General Mobilisation" a few weeks back and new soldiers are being trained up.
It seems to me that "rear guard' forces are supposed to hold Ukraine up long enough for Russia to reorganise and redeploy, but they are not doing that, and Russia seems to be losing ground in areas like Kherson, where they probably didn't expect it.
If the bridge is blown Ukraine will simply find another way across the river, but they are gain ground in the Kherson area, not attacking Kherson city so far.
IMO in the north if Ukraine can drive Russia back to the border they can build strong defences and mix in a few new less experienced troops for the relatively easy task of defending areas where a fully committed Russian attack is unlikely.
Ukraine can free up experienced troops move East perhaps faster than Russia can reorganise and redeploy, part of this is the Ukraine army is naturally more nimble and flexible.
Yes, SAMs are important to reduce the threat from Russian aircraft, but well equipped and well trained troops on the ground make a difference.
Short of using Nuclear, I'm still not convinced Russia has any path to meaningful military victory with 6-12 months, and I'm equally not convinced the Russian economy will be standing in 6-12 months, especially if more is done on sanctions.
Ultimately against a determined enemy, the only way to win is to take and hold ground. That requires lots of troops with the skills to stop an insurgency. The US wars in Afghanistan and Iraq failed because they never put enough boots on the ground. Those who were there were very capable, but there just weren't enough troops.
If the army becomes completely unable to function due to collapse of their supply system, loss of equipment, loss of leadership, and basement level morale, the Russians can keep firing off Iskanders at Ukraine and flying aircraft over Ukraine, but it would just be for spite. There wouldn't be any real military reason anymore.
A couple of things could happen at that point. If their army basically ceases to exist, they might panic about a Ukrainian invasion and pull back their artillery and aircraft in case the Ukrainians try to take their bases.
Another possibility would be with all the troops out of Ukraine, the UN or NATO might declare a no fly zone as a peacekeeping mission. With Russian troops in Ukraine, the Russians could claim that their long range artillery and aircraft are serving in support of their ground forces, but with no troops left in the country they don't have that fig leaf to hide behind. With no army left to speak of, the Russians might do some sabre rattling about it, but would give in. Starting a ground war with the most powerful and best trained force in the world (NATO) would be a dumb move with only shattered elements of an army left, even for Putin.
But then option 3 would be to go nuclear. I don't think Putin would risk it though. I'm sure multiple world leaders have told Putin that going nuclear would unleash NATO. If the EMP from a nuke took out any electronics in Poland or Hungary or radiation from a nuke fell on NATO territory, that could be an excuse to trigger Article 5. And they have probably conveyed that to Putin.
It does look like Russia's rear guards are falling apart and the Ukrainians are rolling them up. The Ukrainians started the TDF (Territorial Defense Forces) at the start of the war. It's a militia basically, but they have performed quite well. They have been involved in some vehicle ambushes and taken out a lot of Russian equipment. Before Ukraine quit reporting their force numbers in early March they already had 60,000 in the TDF.
The TDF is a great force to hold ground and free up the regular army for operations in the south. The collapse of the northern front is creating a race between armies like the collapse of the Russian Empire in 1917 at the same time the US entered the war caused a race. The Germans scrambled to move troops to the western front before the Americans could get there. Nobody expected it, but the Americans won the race and the German transferred troops didn't give them any benefit.
The Russians have further to go moving troops. The Ukrainians have a straight line from around Kyiv to southern areas. The Russians have to pull their troops out into Belerus or across the Russian border, then transfer them around Ukraine. The Russian forces are almost certainly more depleted than the Ukrainian forces and it's certain their morale is vastly lower than Ukraine's. The Ukrainian troops would be coming off a win in the north and will be in high spirits and ready to route Russians in the south.
The Russians from the north are a spent force. Frostbite has been a severe problem. The Ukrainians have found Russians frozen to death, which is another indication of how the Russians have degraded. One thing they used to do very well was winter warfare, but they are losing soldiers to hypothermia and frostbite in the spring!
Their surviving equipment is mostly trashed at this point and their reserve equipment is mostly ready for the junkyard. They will scrounge what they can to give to the units pulled out of the north, but it will likely be much older and both more vulnerable to ATGMs and in many cases broken. To just bulk up their forces they will likely send barely mobile T-64s into combat with the former northern forces.
The Russians are going to be conscripting people over the next 3 months, but it will take some time to get those people ready to go, even if they have the bare minimum of training. I think the Ukrainians will get to the south first and with a stronger force.
