NEW YORK (THE WASHINGTON POST) - Mrs Hillary Clinton and Mr Donald Trump battled over their judgement and preparation to be commander-in-chief at a forum that sounded the starting gun for the final stretch of the presidential race and offered a potential preview of what are expected to be rollicking debates in the weeks to come.

In back-to-back appearances at a forum focused on national security here on Wednesday (Sept 7), Mrs Clinton offered herself as a model of "absolute rock steadiness" on foreign policy while Mr Trump promised to be a disruptive force for improvement, saying that under President Barack Obama, even the military's generals have been "reduced to rubble."

While Mrs Clinton appeared serious and even stilted as she sometimes awkwardly navigated tough questions about her use of a private email server while she was secretary of state and her vote for the Iraq War in the Senate, Mr Trump offered no such restraint, making a series of controversial statements.

He reaffirmed his view that having men and women serve alongside one another is the root of sexual assaults in the military. He said recent intelligence briefings he has received have convinced him that Mrs Clinton and other Obama administration officials did not heed the advice of experts.

And he defended his mutual admiration with Russian president Vladimir Putin, even suggesting that Mr Putin is more worthy of his praise than President Obama.

"Do you want me to start naming some of the things President Obama does at the same time?" Mr Trump said when asked to defend some of Mr Putin's aggressions on the world stage.

He also said that an alliance with Mr Putin would help defeat the Islamic State in Iraq and Syria (ISIS). "Wouldn't it be wonderful if we could work on it together and knock the hell out of ISIS?"

Ahead of Wednesday's forum, Mr Trump delivered a speech in Philadelphia in which he called for a robust expansion of US military capabilities and an end to budget sequestration on defence spending.

"As soon as I take office, I will ask Congress to fully eliminate the defence sequester and will submit a new budget to rebuild our military. It is so depleted. We will rebuild our military," he said during the speech at the Union League of Philadelphia.

"This will increase certainty in the defence community as to funding and will allow military leaders to plan for our future defense needs."

Mr Trump's address represented his most substantive and comprehensive plan on national security to date - part of an ongoing effort to assuage doubts that he lacks a sufficient understanding of policy issues to assume the presidency.

Reading from prepared remarks, Mr Trump spoke about modernising the military's equipment and increasing the size of the armed forces.

Mrs Clinton spent the earlier part of her segment of the forum defending herself regarding the email controversy as well of her handling of classified material, which she insisted she did responsibly.

And she acknowledged that her vote to authorise force in Iraq, while a senator from New York, was a mistake. But she said she had learned from the decision and chided Mr Trump for saying he opposed the war from the outset despite statements to the contrary.

"I have taken responsibility for my decision," Mrs Clinton said. "He refuses to take responsible for his support."

The forum, broadcast by MSNBC and NBC stations and hosted by Mr Matt Lauer of the "Today" show, was billed as a discussion of the most important issues facing the nation's next commander-in-chief.

Mrs Clinton and Mr Trump appeared consecutively before a live audience of active-duty and military veterans - a group that traditionally skews Republican - at the Intrepid Sea, Air & Space Museum in New York at an event hosted by the Iraq and Afghanistan Veterans of America.

The forum was scheduled ahead of three more traditional debates between the two major-party candidates, the first of which is scheduled for Sept 26 at Hofstra University in Hempstead, New York.

Both Mrs Clinton and Mr Trump were asked early in their segments not to attack their opponent - and both drew admonishments from Lauer for ignoring him.

Mrs Clinton, for example, chided Mr Trump for proposing a ban on Muslims entering the country, saying "that is not going to help us succeed in defeating ISIS." And Mr Trump accused her of having a "happy trigger," a suggestion that she is too eager to insert the United States into international conflicts.

The United State's involvement in Iraq was a prominent part of both segments.

Mrs Clinton argued that the war was one area where she and Mr Trump shared positions:

"I think that the decision to go to war in Iraq was a mistake and I have said that my voting to give President Bush that authority was, from my perspective, my mistake," she said. "There was a mistake. My opponent was for the war in Iraq. He says he wasn't, you can go back and look at the record. He supported it."

As he has in the past, Mr Trump insisted that he did not support President Bush's invasion in 2003, despite evidence to the contrary. The businessman brushed off Mr Lauer's attempts to suggest otherwise.

During his segment, Mr Trump also defended a tweet that he posted three years ago that stated the estimated number of unreported sexual assaults in the military and then mused: "What did these geniuses expect when they put men & women together?"

"Well, it is, it is a correct tweet," Mr Trump said when asked about the tweet by Mr Lauer. "There are many people that think that that's absolutely correct ... Well, well, it's happening, right? And, by the way, since then, it's gotten worse."

When asked what he'd learned from his intelligence briefings since becoming the Republican nominee, he said that Mr Obama and others "what our experts said to do."

"And I was very, very surprised," he added. "In almost every instance, and I could tell, I have pretty good with the body language, I could tell, they were not happy. Our leaders did not follow what they were recommending."

Mr Trump also defended his relationship with Mr Putin, saying: "I think I would have a very, very good relationship what Putin and I think I would have a very, very good relationship with Russia."

He denied that mutual admiration between him and the Russian leader would benefit Russia in its dealings with the United States.

"It's not going to get him anything," he said. "I'm a negotiator."

During her segment, Mrs Clinton defended her use of classified information, insisting that she never sent or received any documents on her private email server that were properly marked as classified.

She was pressed with questions on the issue almost immediately forum got underway."Classified material has a header that says 'top secret,' 'secret,' 'confidential,'" she said. "None of the emails sent or received by me had such a header."

The issue has bedeviled her campaign from the start, and one of the questions about it came from a veteran, who noted that if he had handled classified information in a way that was not permitted, he would have been prosecuted and jailed.

"I communicated about classified material on a wholly separate system," she said. "I took it very seriously."

A poll released on Wednesday showed Mr Trump with a sizable lead among active-duty and military veterans. He leads Mrs Clinton by 19 points - 55 per cent to 36 per cent - among voters who are currently serving or have previously served in the US military, according to the an NBC News/SurveyMonkey tracking poll released on Wednesday.

That's a somewhat larger margin that two other recent national polls of military veterans since the Democratic convention. Mr Trump led Mrs Clinton by 14 points in a a Fox News Poll and 11 points in a McClatchy-Marist poll.

The demographics of military veterans align closely with Mr Trump's strongest sources of support; More than 9 in 10 are men, and about 8 in 10 are white. In 2012, among military veterans, Republican Mitt Romney bested President Obama by about 20 points, according to exit polls.

The run-up to Wednesday's forum featured intense jockeying from Mrs Clinton and Mr Trump over which candidate is better suited to lead the military - with both sharply questioning the other's temperament and judgment.

During a campaign stop om Tuesday in Tampa, Mrs Clinton said Mr Trump had "no clue" about national security issues. Her campaign also released a new television ad on Tuesday that used some of Mr Trump's past comments to try to call into question his commitment to military veterans and the sacrifice of military families.

Just minutes before the forum was set to begin, Mrs Clinton tweeted: "Donald Trump has proven over and over again that he's unfit to be our Commander-in-Chief."

On Tuesday, Mr Trump announced that he has been endorsed by 88 retired senior military officials. Not to be outdone, Mrs Clinton released a list on Wednesday showing the support of 95 retired generals and admirals, more than any recent non-incumbent Democrat, her campaign said.

Meanwhile, Mrs Clinton noted that Mr Trump's endorsement figure was more than 400 shy of the last Republican nominee Mitt Romney.