Twelve years ago, four airplanes-turned-missiles shattered a decade-old set of beliefs about the world and our place in it. Twelve years later, we’re still working on a replacement.

Understand that, and you’ll understand our halting, haphazard response to the crisis in Syria — and why Americans are reluctant to follow the president’s lead, inasmuch as they can make it out.

It was tempting, in the years following the Soviet Union’s dissolution in 1991, to believe history had rendered a decision. After a half-century of sorting the world into Us and Them, it was possible to believe Western liberal democracy — the form of government practiced by Us — had won.

In the long run, there is still reason to believe that’s true.

So when terrorists struck in New York and Washington 12 years ago, it was tempting to believe the lands from which they came needed a strong dose of history’s verdict. In the form of democracy. Which we would bring them, like it or not.

There was reason to believe they would like it. What's not to like about freedom, peace, equality, dignity, modernity? Once the strongmen standing in the way were removed, these long-repressed people would welcome us.

We knew just where to start. And while Russia was one of the most vocal critics of the Iraq invasion, it seemed noteworthy that nine other former Soviet republics and seven other members of the Warsaw Pact — a whole lot of Them — joined Us to resume putting an end to history.

Many Iraqis did welcome us, ready to embrace democracy. Many did not.

For all its faults and our trials in midwifing it, today’s Iraq is about the closest thing to a democracy there is in the Arab world. But our years in Iraq, with the highs of ink-stained thumbs on election days and lows of blood-soaked streets many other days, changed us, I believe. We bore a heavy burden and paid a dear price.

Which brings us to Barack Obama, a man elected to be the anti-Bush, the anti-neocon.

Early in his first term, when democratic activists took to the streets in Iran, Obama stood coolly by. When the Arab Spring melted repressive regimes in Tunisia and then Egypt, Obama offered only words, sometimes contradictory ones.

Then came Libya, and his administration’s inclination toward humanitarian intervention couldn’t be resisted any longer.

Obama took us to war there — albeit a late, light, little air war in which we famously “led from behind” — to prevent a mass killing of the rebels seeking to overthrow Moammar Gadhafi. Our intervention turned the tide of the war. Within months, Gadhafi was captured and killed.

But as in Iraq, all did not live happily ever after in Libya. The vacuum of power was filled by Islamic extremists. Libya has collapsed into a humanitarian and economic disaster.

Unlike in Iraq, our troops aren’t there to witness and combat it. But a year and a half after our intervention, on the 11th anniversary of 9/11, terrorists in Benghazi killed our ambassador and three other Americans under still-murky circumstances. This is not the way one treats one’s liberators.

Now Obama comes asking for another late, light, little air war in Syria, and the public is resolutely opposed.

It’s not just that our leaders seem not to have absorbed the lessons of Iraq, Libya and elsewhere. Intervening in Syria appears detached from any broader strategy in that part of the world. Making matters worse, our old adversaries in Moscow — they still view us that way, anyway — are pulling the strings opposite us.

This asterisk interventionism — we won’t mess with you (* unless we do) — holds no appeal for America.

I don’t think most Americans are resigned to a lesser, more passive role in the world. I do think there is much skepticism that the kind of action we are asked to support in Syria is our proper role.

And I think we’ll resume our optimism about shaping the course of human events, and our willingness to do the hard work that requires, when someone points out the clear path forward.