The biggest difference between Ukraine and Afghanistan or Vietnam is the ability of people to leave. The borders to Poland are open, almost anybody left is going to be hostile or a collaborator or deluded.
The fertility rate in Ukraine is 1.23. In Afghanistan in 2001 it was 7.39. Vietnam in 1960 was 6.35. Its much easier to fight guerilla war when you have hordes of young people. Ukraine is old, but so is Russia. Every male killed in this war there is probably the end of male lineage for that family.
That makes Russian position even worse. Because Poland will allow supply of weapons and fighters through its border. Making this a protracted war that Russians can’t win. Look at America and Pakistan, talibans moved at Will between the borders and American forces can’t engage.
Yes. Endless supply from the north swooping into Cambodia around the DMZ and right back into South Vietnam. They moved at-will nearly anywhere they wanted in South Vietnam.
At will... aside from the strategic and interdiction bombing of North Vietnamese trails (officially) and Cambodian trails (semi-officially) and substantial (especially for the 1960s) monitoring of the routes.
You're underestimating the effort and dedication that it took to move sufficient quantities of freight by foot over mountain trails.
"Plan A", this was not. That would have been motorized transport, which was effectively interdicted.
That North Vietnamese logistics was able to smuggle sufficient tonnage to supply even a limited insurgency in the South wasn't inevitable. And probably couldn't have been duplicated by most other military forces.
