We have been shown wide angle aerial photos of vast areas of Mariupol which were totally devastated. Russian troops did not have to do that
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You are making too much of the flattening of Mariupol’ - its just the Orc way of war - they always shell indiscriminately - especially civilians.
There are dozens of towns and villages that only have the outline of the buildings remaining: it something is two feet high, shell it until its is one foot high, then shell that until the rubble bounces at least once. It costs them nothing to do this outside of transportation costs.
They have enough shells to fight WWII all over again with lots remaining to shell all other countries; they have millions on millions of shells stockpiled because they once planned to take over the world.
There are other reasons besides the so called underground resources - which are very limited and not worth going to war for in the first place:
Most importantly is to secure the plains leading directly into Russia to the Urals - this is an indefensible invasion route.
Second is to secure the fresh water supply to northern Crimea, without which the sandy soil becomes salty and uninhabitable - everything in the Crimea depends on this source of fresh water. When Russia lost the Crimea, Ukraine shut this water supply off, rendering the soil in Crimea salty again.
The Hobbits and wizards have been shelling the Donbass for 8 years. I would say it’s their way of war. The even refused Zelenskys direct order to pull back heavy weapons from their positions. It was to his face, on video. Even Zelensky helplessly said “I am the President, you can’t talk to me that way”.
But they did. Right Sector, Svoboda and their Azov guys run the country.