Republican presidential hopefuls sparred over how to stop the so-called Islamic State (IS), in the first debate since attacks in California and Paris.
The national security focus yielded heated exchanges between Senators Ted Cruz and Marco Rubio who clashed on surveillance and immigration policy.
Jeb Bush also sought to revive his struggling campaign by forcefully attacking front-runner Donald Trump.
“You’re not going to be able to insult your way to the presidency,” he said.
Mr Trump was on the defensive early in the debate for his proposed ban on Muslims entering the US, saying, “We are not talking about religion, we are talking about security.”
However the debate quickly expanded to broader issues of foreign policy and national security.
The candidates repeatedly addressed heightened fears of terrorism in the US on the same day an emailed threat shut down Los Angeles’ school system.
(READ more: http://www.bbc.com/news/world-us-canada-35106659)