The North Atlantic Treaty Organization was one of the most successful and effective international institutions of the 20th century.
Constructed as a brilliant compromise between voluntary association and collective endeavour it succeeded beyond all expectations in keeping the world free from tyrannical communist encroachment by the then Soviet Union, and in the end presided at the collapse of the whole Soviet empire and the return of many captive nations to freedom.
But in this second decade of the 21st century NATO faces new and unforeseen challenges that threaten its very existence and purpose.