Columbia subject headings:
Boundaries--Political aspects.
Boundaries--Social aspects.
Harvard subject headings:
Border security--Europe.
Europe--Administrative and political divisions.
Europe--Emigration and immigration
Europe--Foreign relations
Europe--Historical geography
Geopolitics
Princeton subject headings:
Border security--Europe.
Europe--Administrative and political divisions.
Europe--Emigration and immigration.
Europe--Foreign relations.
Europe--Historical geography.
Geopolitics.