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Fmr. DHS Secy. Johnson: Top security threats to U.S. are 'global warming, global warming, global warming'


Former Secretary of Homeland Security joins Andrea Mitchell to evaluate the options to monitor and respond to terror and other security threats that emerge from Afghanistan after the U.S. military withdrawal, and share his thoughts on how the most pressing threats to the homeland have changed 20 years since the 9/11 attacks.