Sometime next month, Russia may begin shipping its S-400 air defense system to Turkey. It is a move that divides NATO, may see the Turkish military kicked out of the F-35 Joint Strike Fighter project, and demonstrates just how central yet divisive high-tech weapons exports have become.

Throughout the Cold War, weapons shipments from both East and West were vital for entrenching alliances and establishing spheres of interest. While some nations — particularly the United Arab Emirates and Saudi Arabia — embraced cutting-edge U.S. technology, many others purchased Russian — hard-wearing, often cheaper equipment that came with wider Soviet diplomatic and economic support.

The world now is rather more complex. A growing number of countries, such as Turkey, Iraq and India, wish to hedge their bets and buy from both. That's understandable — but changing technology brings further complications.