rex, ra_s.t.ra, nation

I understand from Bartleby etyma that the IE root reg- had
lengthened-grade form *re_g-, Indo-European word for a tribal king
and that the root is cognate with Reichsmark, from Old High German
and Latin rex, king, Sanskrit ra_ja_.

Perhaps, historical and comparative linguistics will explain as to
when the concept of 'nation' emerged in IE history.

How is the form, ra_s.t.ra (nation) used in the R.gveda explained?

I request help from members on bibliographical references where this
evolution of polity is discussed further. I tried to search the
archives of the list and could not find messages on this IE root.
Maybe, I have missed something.