Geography
The Role of NATO in Post-Cold War Security Architecture
Quick fact
NATO has conducted more out-of-area operations since the Cold War ended than during its entire Cold War existence, and its membership grew from 16 to 30 countries, yet it has never invoked Article 5 for a direct attack on a member—only for a cyber attack on Estonia (which invoked solidarity, not military action) and for the 9/11 attacks on the US.
Why this is interesting
NATO was created to defend Western Europe from a Soviet attack, yet after the Iron Curtain fell, it didn't dissolve. Instead, it took on new missions—from peacekeeping in the Balkans to fighting in Afghanistan—and kept growing, even as its original enemy vanished.