You write: "Truths are not defined by agreement".
Well, some truths are defined only by agreement.
Every collective belief is defined by agreement
between people. A nation ("Am") is also such a
collective. If a group of people call themselves
and treat themselves as a nation (agreemnet 1) and
also the people around them call them a nation and
treat them accordingly (agreement 2) - then they are
a nation.
Simple, isn't it?
True, this means that there is no dictionary definition
of "nation", because whenever a new group of people
becomes "a nation", the deifnition of "what is a nation"
changes. Life is dynamic. It doesn't happen according
to historians' definitions, and dynamic is also the
meaning of words.