1. If there is one city of that nation closer to a square than
all
other
cities of that nation, then if the state that controls the city is
of that nation, the square belongs to the state that controls
that city. Otherwise the square is neutral.
2. If there are two cities of that nation equally close to a
square than all
other
cities of that nation, and both are controlled by the same side,
then the square
belongs
to that side. If they are controlled by different sides, then:
3. If there are three or four cities of that nation equally close to a square (there are about 20 squares for which this is true) then if all are controlled by the same side, that side owns it. If all sides of that nation controlling one of those cities are allied with all the rest, none of them owns it but they and all nations allied with all of them may deploy there in peacetime. Otherwise the square will not be owned by any state. (Unusual cases can occur when the three nearby states are A, B, and C, A and B are allied, A and C are allied, but B and C are not. Even more unusual cases can occur when there are four nearby cities.)
4. Control of strategic squares in a nation is determined only by cities of that nation. That is, German cities do not affect control of Italian strat squares, and vice versa (this is a particularly important point in the Savoy/Piedmont near Geneva) and similarly for any other nation with two states.