Trading was often between similar groups, such as islamic countries trading with other islamic countries was often very impactful to the success of a concept. In North Africa, many countries very quickly converted to Islam, in order to build cultural/trade relations with other countries, and also harmed nearby countries that followed non-Islam religions, such as Axum, which was Christian. (info collected from 8.7, page 345) The CPSU would do very little trade outside of the country, and whenever they would choose to, it would most often be with other Communist countries, such as China or Satellite states in Eastern Europe, which would be communist countries under heavy influence from the Soviet Union.
There were issues with sustaining a military. One of the reasons that Rome fell was due to economic issues with heavy taxes in order to maintain their army; also, they were hiring mercenaries, who were only in it for the money, not out of caring for the country. (collected from 6.2, page 209) One issue causing the collapse of the Soviet Union was due to their extreme investments in their military, putting 10-20% of their GDP as military spending(For context, the US currently spends less than 4%), and letting other parts of the country fall apart; soldiers also felt that the extreme involvement with the Afghan War was against their interests, and many people didn't feel connected to the interests of the Russian leaders.