Sunday on 'This Week': Former Joint Chiefs Chair Adm. Mike Mullen

— -- As 2017 comes to a close, former Chair of the Joint Chiefs of Staff Adm. Mike Mullen discusses the global challenges facing President Donald Trump in the coming year, Sunday on “This Week.”

Plus, the Powerhouse Roundtable analyzes the major events of the past year and looks ahead to 2018, with ABC News political analyst Matthew Dowd, FiveThirtyEight senior political writer Perry Bacon Jr., Washington Post national correspondent Mary Jordan, and USA Today Washington bureau chief Susan Page.

And former NYPD commissioner and ABC News contributor Ray Kelly, and former FBI agent and ABC News contributor Brad Garrett discuss security preparations as millions of Americans plan to gather for New Year’s Eve celebrations around the country.

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