Adjectival form of "foreign policy"?

Solution 1:

Diplomatic is the word you want here. Dictionary.com:

adjective
1. of, pertaining to, or engaged in diplomacy:
diplomatic officials.

Diplomacy:

noun
1. the conduct by government officials of negotiations and other relations between nations.

2. the art or science of conducting such negotiations.

So your example sentence would be completed as follows:

There were contrasting political, social, and diplomatic systems.

Solution 2:

Since you've clarified you're looking for a word which describes "military relations", perhaps "martial" would suit:

martial: relating to fighting or war.