Taking ground is secondary, incidental at this point. It will become primary later.
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For the russians, taking ground is primary. Or anyhow that explanation best explains their actions.
But what do you mean by the above?
Russia has said the entire way it has two goals in Ukraine. Assimilate areas where Russian speakers live and demilitarize Ukraine. They are working on both as they are connected. Once Ukraine’s army is attritted taking land will be easy and won’t require heavy Russian losses.