Democratic presidential candidate Hillary Clinton says "steadiness" is the most important characteristic a president must have.
Her Republican rival Donald Trump says voters do not have to worry about his temperament or judgement.
Clinton spoke first Wednesday night at an NBC News Commander-in-Chief Forum held aboard the Air Force carrier Intrepid in New York City. In a likely preview of their three upcoming debates, Both Clinton and Republican rival Donald Trump were appearing separately on the same stage, answering questions about national security.
They agreed not to use the forum as a place to attack each other but occasional verbal jabs crept into the discussions.
Clinton said a president has to be someone who listens, evaluates what he or she is being told and said temperament and judgement are the keys.
She called military force a last resort and that learned that the war in Iraq was a mistake, admitting that she also made a mistake supporting it when she was a senator. Clinton said the U.S. will not put ground forces on the ground in Iraq "ever again."
She said the nuclear deal with Iran put a lid on its nuclear program, and said she would enforce it "to the letter." Clinton said the U.S has to squeeze more support from its Arab allies in the war on Islamic State.
Trump told the forum he would be "very very cautious" when deciding whether to send Americans into battle, accusing Clinton of having a "happy trigger."
He said he was "shocked" to hear in classified national security briefings given to major presidential candidates that Clinton, President Barack Obama and Secretary of State John Kerry did not follow recommendations of intelligence experts.
Trump was unashamed about his praise for Russian President Vladimir Putin despite tensions between the White House and Kremlin. He told the forum he would have a "very good relationship" with Putin and many other foreign leaders.
Clinton and Trump will hold their first face to face debate on September 26. It is the first of three scheduled debates in the weeks before the November 8 election to pick the successor to President Barack Obama.
Earlier Wednesday, Trump unveiled his plans for the military.
"I'm going to make our military so big, so powerful, so strong, that nobody, absolutely nobody, is gonna mess with us," Trump declared on his campaign website.
He later told an audience in Philadelphia that he would give U.S. commanders 30 days to come up with a plan to defeat Islamic State, after boasting this week that he has his own "secret plan."
Clinton scoffed at Trump's claim, saying "the secret is, he has no plan."
She said American voters "know they can count on me to be the kind of commander in chief who will protect our country and our troops, and they know they cannot count on Donald Trump. They view him as a danger and a risk."
Trump said Clinton's time as U.S. secretary of state from 2009 to 2013 left the Middle East in "more disarray than ever before.”
"She's trigger happy and very unstable," Trump said. He also accused her of being "reckless" in the way she handled her emails when she was secretary.
U.S. investigators concluded Clinton was "extremely careless" in her handling of national security materials, but that no criminal charges were warranted.