If NATO intervened, it might improve Russian moral and it's best to support and arm highly motivated Ukrainians. A big part of their war plan is a very clever PR plan and being humanitarian is a big part of this. Russia has called up old men in the reserves.
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Poor Planning, Low Morale, Weak Supplies Corrode Russian Invasion From Within
Rob Lee, senior fellow at the Foreign Policy Research Institute, talks about the incompetence and under performance of the Russian military in pursuing Vladimir Putin's invasion of Ukraine, slowing what had apparently been planned as a quick operation.