President Obama concedes that the Islamic State in Iraq and the Syria (ISIS) does not pose a direct threat to our country but argues that one day it might. That is the core of the case for the new war in the Middle East that Obama announced this week, although it's easy to forget amid all the other rationales.

Most of those rationales have little or nothing to do with U.S. national security, and some are breathtakingly broad. In a recent London Times essay, Obama and British Prime Minister David Cameron declare that "we will not waver in our determination to confront ISIL" (a.k.a. ISIS) because "we have a real stake in making sure that [our children and grandchildren] grow up in a world where schoolgirls are not kidnapped, women are not raped in conflict and families aren't slaughtered because of their faith or political beliefs."

Those are all terrible things, but the U.S. government has neither the mandate nor the ability to prevent them, as Obama himself occasionally acknowledges. "Not every regional terrorist organization is automatically a threat to us that would call for a major offensive," he told NBC's Chuck Todd on Sunday. "Our goal should not be to think that we can occupy every country where there's a terrorist organization."

So ISIS is "a threat to us"? Not quite. "We have not seen any immediate intelligence about threats to the homeland from ISIL," Obama emphasized. "That's not what this is about. What it's about is an organization that, if allowed to control significant amounts of territory, to amass more resources, more arms, to attract more foreign fighters…can be a serious threat to the homeland."

You may or may not find this preventive/pre-emptive argument for war persuasive. It seems to me that ISIS is, first and foremost, a problem for its various regional adversaries, including Iran, the Persian Gulf states, rival rebel groups and the Assad regime in Syria, and the Kurds, Shiites, and moderate Sunnis of Iraq.

The horrifying videos showing the beheading of two American journalists, which are probably the main source of outrage feeding support for war, show that ISIS is ruthless and barbaric. They do not show that ISIS is a problem America has to solve.

Sen. Rand Paul (R-Ky.), ordinarily cautious about foreign intervention, alluded to those videos when he announced his support for a wider military campaign against ISIS. "This is something we can't take," he said on Fox News last week. "We're not gonna let our enemies behead our journalists." With American journalists working all over the world, protecting them seems like a dangerously open-ended justification for war.

Paul also cites "the plight of massacred Christians and Muslim minorities." Secretary of State John Kerry likewise mentions ISIS's "genocidal agenda," along with its "beheadings, crucifixions and other acts of sheer evil," as reasons the U.S. should wage war against the group.

But the world is full of evil, and no country, not even the United States, can hope to eradicate it. Trying to do so is dangerous because of the potential for unintended consequences.

Consider Iraq and Libya, where chaos prevails not just despite but because of U.S. meddling. Given these and many other examples of foreign adventures gone awry, it is quixotic to believe that the U.S. can intervene in Syria's civil war in just the right way, attacking ISIS without shoring up the brutal Assad government or magnifying the terrorist threat to our country.

Supporters of war against ISIS keep citing the group's outrageous crimes—which are not, strictly speaking, relevant to the question of whether it poses a threat to the United States—because their defensive justification is weak, based on speculation about the enemy ISIS could become. "When in doubt, stay out" is a good rule of thumb for dropping bombs in other countries, because the world is complicated and politicians never know as much about it as they think they do.