Twelve years into the American intervention in Iraq, the things opponents — yes, there were opponents to George W. Bush’s invasion, including me — warned about have come true: an Iraqi civil war, spreading instability in the region; the division of Iraq into ethnic and religious strongholds; the rise of extremism; the increase in Iran’s power; the loss of American credibility; thousands of lives and trillions of dollars lost in a desert quagmire.

Historians and commentators on both the left and right now routinely refer to the 2003 invasion as the worst U.S. foreign policy decision in at least a century, but you don’t hear that kind of blunt talk from politicians. Many of them have their fingerprints on this disaster, which has played out through two presidencies.

This reluctance to face the full scope of the failures in Iraq extends to the presidential candidates, and that’s a problem. Presidential campaigns ought to be the place where blame is cast, ideas are aired and policy choices are debated. Anyone who aspires to the White House in 2016 ought to have opinions on the Iraq experience — not least the brother of the president responsible for its first six years and the secretary of state during its next four. But so far none of the candidates have had anything interesting to say about it.

The questions asked of the candidates have been as unsatisfying as their answers. Jeb Bush fumbled the first question, a lame hypothetical about whether, knowing what we know now, he would have supported his brother’s decision to invade. That was a good question for 2004, not 2015.

A more relevant question is whether the Obama administration should have pulled out all U.S. combat troops by the end of 2011, as required under the agreement his predecessor signed.

Another qood question: After the U.S. spent a decade training the Iraqi Army and billions equipping it, why did so many units fall apart in the face of ISIS attacks?

This is important going forward. Training other countries’ armies so U.S. combat forces can leave has been a foreign policy staple for decades. But it didn’t work in Vietnam, hasn’t worked in Iraq and isn’t working in Afghanistan.

It’s also the heart of Obama’s strategy for Syria: Applying the magic of U.S. military training and equipment to an as-yet-unnamed force of “moderate” Syrian rebels. Does anyone really think that will work?

The failure of the training is a question for the generals as well as the candidates. Their answer, delivered by Defense Secretary Ashton Carter last week, is to blame Iraq’s politicians. “The Iraqi forces just showed no will to fight,” he said.

An army must have a nation to fight for. Building that nation has always been the biggest challenge in Iraq, which has been an unwieldy amalgam of tribes since European powers put it on the map. But after a dozen years of nation-building, Iraq is effectively divided into Shia, Sunni and Kurdish zones. Each faction has its own militias, all of whom fight with more conviction than the Iraqi army.

A half-century of American interventions abroad should have taught us that toppling a regime is far easier than installing one that can unite a fractured country.

Which brings me to a question for Hillary Clinton. In March 2010, Iraq elected a new parliament, with the party led by Ayed Allawi scoring a narrow victory over Prime Minister Nouri al-Maliki. Allawi’s party was secular, nationalist and won more votes in the Sunni-dominated western provinces. As retired Gen. Daniel Bolger notes in his book, “Why We Lost,” U.S. officials, including Ambassador Christopher Hill and VP Joe Biden, kept Maliki in power despite having lost the election.

Maliki created a Shiite Army only committed to defending Shiite regions. Sunni leaders, disenfranchised in Baghdad, turned to ISIS. No one knows if things would be different today had Allawi been allowed to form a government. But I sure would like to know what part then-Secretary of State Clinton had in the decision to tilt toward Maliki.

In the campaign to come, we’ll hear a lot about the importance of asserting American “leadership” abroad. But leadership is often just another word for intervention: more troops, more regime change, more training and arming of foreign fighters, more sanctions imposed and red lines drawn to get other governments to do what we want.

Let’s ask the candidates what lessons they’ve learned from America’s intervention in Iraq. Because if that’s the kind of leadership they have in mind, an awful lot of voters won’t want to follow.

Rick Holmes, opinion editor for the Daily News, can be reached at rholmes@wickedlocal.com.