In May 2010, the Cooperative Cyber Defence Centre of Excellence and the Swedish National Defence College hosted the Baltic Cyber Shield (BCS) international cyber defense exercise (CDX). For two days, six Blue Teams from northern European government, military and academic institutions defended simulated power generation companies against a Red Team of 20 hostile computer hackers. The scenario described a volatile geopolitical environment in which a hired-gun Rapid Response Team of network security personnel defended Critical Information Infrastructure (CII) from cyber attacks sponsored by a non-state terrorist group. This article covers the origin and evolution of CDXs, and it describes the design, goals and lessons learned from BCS 2010.