"As US makes this transition to what comes after the post-9/11 era, we are only beginning to see the dramatic shifts underway that will define our future and shape our interactions in the world and require our national security institutions to adjust and to adapt. This is a story of history, of mankind, adaptation and adjustment," Hagel told a US think tank.
"Chief among these 21st century trends are shifting geopolitical centres of gravity, reflecting the astounding diffusion of economic power and sweeping demographic change. China, India, Brazil and Indonesia are all helping reshape the global economy," Hagel said at the Centre for Strategic and International Studies, an eminent American think-tank.
"And as Latin America and Africa develop and strengthen, they, too, will be important leaders in helping build a secure and prosperous 21st century world," Hagel added.
He asserted that cyber activists, terrorists, criminal networks and non-state actors are also playing a role in defining the international system, new structures of governance and power that is emerging as the world's population becomes more urbanised, mobile, and technologically advanced, bringing new standards and expectations.
He said the challenge of terrorism has evolved as it has metastasised since 9/11.
Hagel noted that this has required and will continue to demand unprecedented collaboration with partners and allies on counter-terrorism efforts.