President Obama would have been rocked the past few months by five things. One is the building criticism from left and right about his high need for relaxation—playing golf while the world burns. Another is that he misread the significance and public power of the beheadings of American journalists. Third, he has been way out of sync with American public opinion on Islamic State, which must be all the more galling because he thought he knew where Americans stood on the use of military force. Fourth, with his poll numbers declining (32% approval for his handling of foreign policy, according to The Wall Street Journal and NBC), it has probably occurred to him that he is damaging not only his own but his party's brand in foreign affairs. (Yes, George W. Bush did the same to his party, but Mr. Obama was supposed to reverse, not follow, that trend.) Fifth, he surely expects he is about to take a pounding from the antiwar left.

Most immediately interesting to me is the apparent change of mind by Americans toward military action in the Mideast. The president's long-reigning assumption is that a war-weary public has grown more isolationist. But, again according to the WSJ/NBC poll, more than 6 in 10 back moving militarily against Islamic State. Politicians and pundits believe that this is due to the gruesome, public and taunting murders of the U.S. journalists—that Americans saw the pictures and freaked out, that their backing of force is merely emotional.

I think they're missing a big aspect of this story.

A year ago the American people spontaneously rose up and told Washington they would not back a bombing foray in Syria that would help the insurgents opposed to Bashar Assad. That public backlash was a surprise not only to the White House but to Republicans in Congress, who were—and I saw them—ashen-faced after the calls flooded their offices. It was such a shock to Washington that officials there still don't talk about it and make believe it didn't happen.

Why was there such a wave of opposition? In part because Americans had no confidence their leaders understood the complications, history and realities of Syria or the Mideast. The previous 12 years had left them distrusting the American foreign-policy establishment. Americans felt the U.S. itself needed more care and attention. By 2013 there was a new depth of disbelief in Mr. Obama's leadership.

But there was another, powerful aspect to the opposition.

Evangelical Christians and conservative Catholics who would normally back strong military action were relatively silent in 2013. Why? I think because they were becoming broadly aware, for the first time, of what was happening to Christians in the Middle East. They were being murdered, tortured, abused for their faith and run out of the region. And for all his crimes and failings, Syria's justly maligned Assad was not attempting to crush his country's Christians. His enemies were—the jihadists, including those who became the Islamic State.

In the year since, the brutality against Middle Eastern Christians, and Islamic State's ferocious anti-Christian agenda, has left many Christians deeply alarmed. Jihadists are de-Christianizing the Mideast, where Christianity began.

An estimated two-thirds of the Christians of Iraq have fled that country since the 2003 U.S. invasion. They are being driven from their villages in northern Iraq. They are terrorized, brutalized, executed. This week an eyewitness in Mosul, which fell to Islamic State in June, told NBC News the jihadists were committing atrocities. In Syria, too, they have executed Christians for refusing to convert.

In roughly the past 18 months, all this has broken through in Christian communities, largely by way of Christian media, including Catholic news services and the Baptist press. The story has been all over social media. Pope Francis has denounced what is happening; the Vatican is talking about just-war theory.

Rep. Chris Smith, the New Jersey Republican who chairs the House Foreign Affairs Subcommittee on Global Human Rights, this week called what is happening "a genocide."

"It is a global phenomenon, but dramatically in the Mideast," he said.

I told him I thought the journalists' beheadings had put a public picture on a crisis of which Christians in America have now become aware.

"An emphatic yes, with exclamation points put after it," he replied.

No one—at least not the United Nations or other international bodies, and not the administration—seems to be keeping official records. Mr. Smith suggested that when people don't really want you to know the scale of a problem, they don't gather the numbers. He has pressed both the U.S. government and the U.N. for statistics and specifics—how many Christians have been killed, abused, sent fleeing and from where. "It's all, 'I'll get back to you.' When they do, it's threadbare answers that don't say a whole lot."

The anguish and indignation of American Christians at what is being done, by Islamic State, to their brothers and sisters in faith is surely part of the reason Americans are backing U.S. action against the terror group.

It would surely also be a misreading of the polls to announce the American public is suddenly "interventionist." There is no reason to believe they have any appetite for romantic or aggressive forays into invasions, occupations or nation-building efforts. What they want to do—and they wanted to do it last month—is respond to a group that is unusually evil, even by Middle Eastern standards.

There is also no reason to infer from the polls that Americans hold to the illusion that moving on Islamic State would create new order and peace in the Mideast. Those illusions tend to live more in Washington than on-the-ground America. If Islamic State is hit hard enough, it may be killed, but nothing else will be fixed. The Mideast will continue in brutal chaos, but Islamic State, as Islamic State, will be done or at least damaged, and surely that is worth something. At the very least a message will be sent.

If the president were a more instinctive man, or rather if his natural instincts were more in line with those of your average American clinger, he would have moved quickly, sharply and without undue drama. He would have bombed Islamic State when it was a showy army in the field, its fighters driving stolen armored vehicles down highways in the sand, in their black outfits, with their black flags. They are not terrorists hiding in holes and safe houses. They are not doing Internet showbiz from caves, they are seizing and holding territory. They say they are the caliphate, and they intend to expand. They are killing and abusing many, not only Christians. They are something new and deadly.

My guess is two things are not acceptable to the American people. One is the full-scale commitment of scores of thousands of troops to invade and occupy a country. The other is a diffident, confused, unfocused, unserious campaign.

The American people are not suddenly recommitted to a decadeslong drama. They do want to see bad guys taken out. Their timetable, I suspect, would be "Let's start last month."