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, US cities do not affect control of Mexican strat squares, and vice versa.