Why is 'là' needed here with an adjective? Is 'là' always used with 'vô nghĩa'?
My guess is to emphasize that this is referring to all wars. Without "là" I think this would translate to "The meaningless war".
chiến phi là phi nghĩa
No, "phi nghĩa" is different to "vô nghĩa". "Vô nghĩa" means meaningless, "phi nghĩa" means rightless.