Recently Ukraine has started to move south in two thrusts, one headed for Polohy and the other further east nearer to Donetsk. These advances might just be opportunistic local commanders who are simple taking lightly guarded areas. Or they might be something greater. If they end up being part of a larger move by Ukraine, what are these advances about?
If the advances go no further than what we already see, then they haven’t accomplished much directly. They might be diversionary to force Russia to move troops to the area. But as is, they don’t do much more.
However, if Ukraine can move another 10-15 miles south on the Eastern side to take the railroad near Volnovakha they can cut the supply coming to this area by railroad from Mariupol. And if they can push 10-15 miles past Polohy and reach Novopoltavka they can cut the railroad supply from the western side. This would leave all Russian troops between the two advances with truck supply only. This alone would probably induce the Russians to withdraw those troops closer to the coast. The Polohy side is the more important of the two as Ukraine can use the N/S railroad in Polohy for their own supply. The railroad on the eastern advance near Donetsk is currently under Russian control to the NE and would not be immediately useful for supply without a Ukrainian push to take the railroad there.
Simply pushing far enough south to cut the E/W railroad here would be of significant political significance. The point of the Russian “land bridge to Crimea” is to increase transportation options to the peninsula. Not having full control of the only E/W railroad reduces the value of that. Granted, the Russians with time could build a new E/W railroad further south, but it would be an added cost. I say political significance and not tactical or strategic, as it’s currently unclear if the Russians can use it the whole way due to it running close to Ukrainian controlled territory near Donetsk. I have no idea if Russia can safely get trains through that stretch but it seems too close for comfort.
Now what if Ukraine can keep pushing all the way to the Sea of Azov? Taking the cities of Mariupol and Berdiansk would certainly be moral boosters and potentially life-saving for Ukrainians still there. But from a military perspective they have more value to Russia than to Ukraine. Ukraine can’t really use them as ports due to being too dangerously close to Russian naval assets. So the main military benefit of the ports themselves would be denying Russia access to them.
But Ukraine doesn’t even have to take the cities themselves to deny the Russians usage of the ports. All Ukraine needs to do is get to a suitable section of coastline with sufficient roads for supplies and set up Harpoon/Neptune batteries. These batteries, if they could be protected, would threaten all Russian shipping in the Sea of Azov. The major port of Rostov-on-Don would no longer have safe access to the Black Sea. All Russian held Ukrainian ports on the Sea that the Russians are using for resupply would be in severe danger of their shipping being sunk. Depending upon where they could get to on the coast, the strait of Kerch and its bridge would be roughly 100 miles away putting it in range of some weapon systems. Crimea could be cut off from land supply routes.
That’s not the only benefit for Ukraine of cutting a path to the Sea. Assuming the Kerch Bridge remains usable, Russian forces going from Kherson to Donbas, or vise versa, would need to travel the much longer trip through Crimea and across the Kerch bridge rather than the closer roads through Zaporizhzhia Oblast. This would greatly extend the Russian disadvantage of external lines of communication. Should the bridge become unusable, then Russian forces wanting to change from one side to the other would have to take troop transports through the Black Sea. Say Sevastopol to Sochi or Novorossiysk.
And we’re not done here. Should Ukraine make it to the Sea of Azov, Russia would need to defend the Donetsk to Mariupol area. Should Ukraine be able to move quickly enough they could be able to push through the pre-war fortification area before Russia might get significant forces redistributed there. And on the Western side, Ukraine would be threatening Melitopol and even further threatening to cut off Kherson from behind. If Ukraine could move fast enough (unlikely but in the realm of possibility) they might even be able to rush to Crimea before Russia could redeploy significant forces there. Again unlikely, but it would force Russia to move those forces there.
All of this would require major investment in a counter offensive by Ukraine that we have yet to see emerge. So I’m not saying these things will happen. But if Ukraine does put sufficient forces on the offensive to get to the Sea of Azov they would gain some significant advantages by doing so.