The largely economics-free framing of the foreign policy questions versus the 'how-will-you-pay-for-it' framing of the healthcare questions is getting some scrutiny on social media.

The moderators have moved on to healthcare - the star issue of Democratic debates past.

Sanders is a proponent of Medicare for All - a single-payer health system plan he spearheaded. It's been criticised by more moderate Democrats, who say it's too expensive.

Warren says she'll protect the Affordable Care Act (Obamacare) while moving towards Medicare for All - but she won't raise taxes on middle-class families and will reduce drug costs.

Biden says he is for expanding and fixing Obamacare - which extended insurance coverage to many Americans with state-run marketplaces.

Klobuchar points out that a majority of Democrats in Congress don't want Medicare for All. She's for a nonprofit public option.

"I think you should show how you're going to pay for things, Bernie," she says.

• Medicare for All: Can Bernie Sanders overhaul US healthcare?
• Obamacare: Has Trump managed to kill off Affordable Care Act?

Image source, Getty Images

The Warren-Sanders dispute has thrust gender into the spotlight in this debate, far more than in the previous six.

Whether or not Sanders told the Massachusetts senator - as reported - that a woman couldn't win against Trump, Warren insists that the audience shouldn't "deny that the question's there". She compares it to questions in the past over whether a Catholic or an African American could win the presidency.

"Women candidates have out-performed men candidates" since Trump has been in office, Warren says. Indeed, 44% of female Democratic challengers won their primaries in 2018, compared to 21% of Democratic men, according to an NBC analysis.

A record 102 women are serving in the 2018 intake in the House of Representatives, and pundits argue that women could hold the key to the 2020 election.

In 2016, a majority of suburban white women voted for Trump, but since then their support for the president has seen the biggest slip.

There are some signs they are becoming more Democratic and more politically engaged in 2020. According to OpenSecrets, a political finance tracker, more than 1 million women have donated to 2020 presidential candidates already.

The problem for Warren, though? The top candidate receiving funds from women is Senator Sanders.

Earlier in the campaign, the BBC's Cut Through The Noise show on Facebook chased candidates around Iowa to ask if America is ready for a female president.

Politically minded people have gathered at watch parties being held at Washington DC bars.

BBC reporter Helier Cheung went to Busboys & Poets, where the biggest reaction so far was to the Sanders and Warren being questioned over reports he said he didn’t think a woman could beat Trump.

A few people groaned as Sanders denied making the remark, and much of the room erupted into whoops and claps after Warren retorted that the men on stage with her had collectively lost 10 elections.

Image source, Getty Images

The moment we were waiting for. In the sharpest clash so far, Elizabeth Warren and Bernie Sanders are asked about claims made yesterday that Sanders told Warren in a 2018 meeting that a woman could not win this election.

Sanders, who vehemently denied making the remark, goes first. He says he offered to let Senator Warren run first in 2015, and when she did not, he ran.

"Hillary Clinton won the popular vote by 3 million votes," he says. "How could anybody in a million years not believe that a woman could become president of the United States?"

He adds that if any person on the stage wins the nomination (though he hopes it will be him) he will "do everything" in his power to make sure they are elected over Donald Trump.

Image source, Getty Images

Warren replies, and she comes out swinging

"Bernie is my friend and I am not here to try to fight with Bernie," she says, before going on:

"Can a woman beat Trump? Look at the men on this stage - collectively they've lost 10 elections. The only people who have won every election they have been in are the women. Amy and I are the winners."

Some early responses from Republicans tuned in to the debate.

"Joe and I have a fundamental disagreement. In case you hadn't noticed," says Bernie Sanders, after Biden defends US global trade deals.

Sanders says the current approach to trade is just to "increase the profits of large multi-national corporations".

He adds that he is "sick and tired and will not tolerate" US companies that shift jobs abroad after accepting help from the federal government.

Biden responds: "We need corporate responsibility," but adds that enforcement mechanisms are necessary, and should exist in multi-national trade agreements.

Warren adds that companies that do business in the US should also care about environmental standards and other issues beyond their profit margins.

There is daylight between the positions of the two progressive senators, Warren and Sanders, over the USMCA - a proposed new North American trade deal negotiated by the Trump administration as a replacement for Nafta.

Sanders has taken a hard line position and says he would not support it. Warren says she believes that the new deal "is a modest improvement" on Nafta.

Klobuchar and Buttigieg, the two centrists, both say they would support approving the deal.

"We need a big trading block to take on China," says Klobuchar.

The deal "has been improved, it is not perfect", says Buttigieg.

Asked about his position, Biden says: "There will be no trade agreement signed in my administration without environmentalists and trade unions at the table."

Biden says he would not negotiate with North Korea without pre-conditions, reminding voters that the isolated dictatorship has said that "Biden is a rabid dog and should be beaten with a stick".

"Other than that you like him, right," jokes Bernie Sanders.

The assassination by the US of Iran's most powerful general has caused tensions between the two countries to soar, raising fears of all-out war - and it's a key topic in tonight's Democratic debate.

But what lies at the root of the crisis?

We've explained the story in 400 words.

The Trump campaign spokesman has tweeted to criticise the candidates' performances so far.

In a tweet Tim Murtaugh wrote: "Not one of these Democrats is describing the United States in terms of strength. Every one is projecting weakness."

As CNN notes, this is the fifth US presidential campaign cycle in which the Iraq war has been debated by the candidates.

The candidates are asked about whether they would take military action without congressional approval (as Trump did with a controversial air strike on Iranian general Qasem Soleimani).

Biden is asked whether his administration would differ from the Obama administration, which the moderators say took military action without asking Congress for permission.

Biden doesn't quite answer, and says the Obama administration had authorisation - a prior authorisation that was passed by the US Congress, House and Senate, "that was the authority".

Buttigieg says that authorisation "needs to be replaced", to which Biden agrees.

Buttigieg adds: "When we lost troops in Niger, members of Congress didn't even know we had troops there."

Warren says she would require congressional approval for all military action - except in the case of an imminent threat.

Image source, Getty Images

Senator Warren says the US must not rely on its military to solve its problems in the Middle East.

"Stop asking our military to to solve problems that cannot be stopped military," she says, adding that America must do more to work with its allies to exert diplomatic and ecomoic pressure on its rivals.

"We need to get our combat troops out. They are not helping to keep our troops safe throughout the region," she adds. "We've turned the corner so many times we're going in circles".

She criticises Trump's decision to deploy more troops after the killing of top Iranian general Qasem Soleimani.

Buttigieg, an Afghan war veteran, says America must end its "endless commitment of ground troops" to the Middle East.

Senator Sanders, who voted against the 2003 US invasion of Iraq, says the "American people are sick and tired of endless wars that have cost us trillions of dollars".

Biden has been leading in national polls since announcing his candidacy, but a key Iowa poll released this week has put Sanders in pole position to win the first-in-the-nation primary contest.

According to the Des Moines Register/Mediacom/CNN Survey Sanders has 20% support among Iowa primary voters. Elizabeth Warren is second, with 17% support, Pete Buttigieg has 16% and Joe Biden has 15%.

The highly-respected poll has only failed to accurately predict the Iowa caucus winner twice in the past 30 years.

Addressing the question of experience to be commander in chief, Pete Buttigieg, who is a veteran of the war in Afghanistan, says security challenges rapidly and dramatically changing.

"It's not just conventional military challenges, not just stateless terrorism, but cybersecurity challenges, foreign interference in our elections," he says. "It's going to take a view to the future as well as the readiness to learn from the lessons of the past.

"And for me, those lessons of the past are personal."

Amy Klobuchar, who clashed with Buttigieg in a previous debate after criticising his experience, says that while she respects the former mayor's military service, her experience as a senator is a better fit to take on the role of commander-in-chief.

In her first answer, on foreign policy, Senator Elizabeth Warren says the most important job of any American president is to "keep America safe". She says the US is spending too much on its military.

The Massachusetts senator says she has travelled as a lawmaker to Afghanistan, Iraq, Jordan, and South Korea.

She also calls the influence of the military industrial complex "corruption, pure and simple", and promises that as president she will "not let the defence industry call the shots".

Image source, Getty Images

The first question hints at the focus on foreign policy to come: Why are you the best to be commander in chief?

Bernie Sanders goes first, citing his ability to work across the aisle.

• "I'm able to bring people together where we solve conflicts over the negotiating table, not through military efforts."
• Later, attacking Biden's vote in favour of the Iraq war years ago, Sanders called that war the "worst foreign policy blunder in the modern history of this country"

Next up, former vice-president Joe Biden.

Biden acknowledges his mistake with Iraq and adds that he knows what it's like to send a son or daughter to war.

• "That's why I do so very very reluctantly...I think my record overall on everything we've done - I'm prepared to compare it to anyone on this stage."

Anthony Zurcher
BBC North America reporter, in Iowa

Image source, Getty Images

Debates are about conflict and contrast. That’s what makes them more interesting – and more illuminating – than the single-candidate town hall forums and panel discussions that have dotted the political landscape for more than a year.

With that in mind, here are some of the possible clashes that could take place in tonight’s Des Moines debate:

Elizabeth Warren versus Bernie Sanders: Is the progressive non-aggression pact over?

Sanders versus Joe Biden: Will Sanders go after Biden’s support for the 2003 Iraq War, the 2005 bankruptcy reform bill and past calls to cut public welfare?

Pete Buttigieg versus Warren: They clashed over the role of money in politics last time--Warren derided the former South Bend mayor for holding a fundraiser in a "wine cave". Will this be round two?

Amy Klobuchar versus the field: Can the fifth-place candidate find a way to burst into the top tier before it’s too late?

The six Democratic candidates participating in tonight's debate have taken the stage.

Five minutes to go until it all kicks off.

• Biden served for decades in Washington as the senator from Delaware before he was tapped to be vice-president by Barack Obama in 2008
• His career has partly been shaped by personal tragedy; his wife and baby daughter died in a car crash in 1972 and his son died of brain cancer in 2015
• He and his son Hunter are front and centre of the impeachment effort against Trump, after the Trump asked Ukraine to investigate the Biden family for corruption - an act that Democrats have called an abuse of power