Adjectival form of "foreign policy"?

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.

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

martial: relating to fighting or war.

strategic:

adjective

1. relating to the identification of long-term or overall aims and interests and the means of achieving them. "strategic planning for the organization is the responsibility of top management" synonyms: planned, calculated, deliberate; More antonyms: random designed or planned to serve a particular purpose. "alarms are positioned at strategic points around the prison"
2. relating to the gaining of overall or long-term military advantage. "Newark Castle was of strategic importance"