Nations that have little interaction with one another have little knowledge of one another’s needs and problems.
Setup
If two nations have little interaction with each other, then they have little knowledge of another's needs and problems.
Sympathy and justice depend largely on understanding the needs and problems of others.
In other words, if you don't understand the needs and problems of others, it will be hard to achieve sympathy or justice with them.
Anticipate / Goal
We know that the fill in the blank is going to synthesize multiple ideas. There are only two ideas here, so we have to figure out what we get by combining them.
Usually the key to synthesis is that two claims have some overlapping idea: in this case, the overlapping concept is knowledge/understanding of another's needs and problems.
We can imagine these ideas chaining together to give us something like this sort of chain: if two nations don't interact much, then won't understand other's needs / problems, and so sympathy / justice will be difficult.
If we wanted to pretend this was a true conditional, it would look something like this: little → little understanding → no sympathy interaction of needs / problems or justice
In that case, a contrapositive chain would sound like: Sympathy and justice require understanding each other's needs/problems, which requires interacting a lot with each other.