With the backdrop of terrorist attacks in Paris and San Bernardino, the fifth Republican presidential debate was the most serious to date. Led by a more spirited Jeb Bush, it was still fiery because of the numerous attacks on Donald Trump. But, highlighted by exchanges between Ted Cruz and Marco Rubio, there was substantive disagreement on foreign policy and national security. Echoing Rand Paul, Trump and Ben Carson, Cruz said: "We need to focus on American interests, not on foreign interventions." Chris Christie and Bush favored a no-fly zone in Syria; Rubio and John Kasich called for ground troops. Paul warned of World War III. The must-watch moments:

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