Fox News host Bill O'Reilly announced on Tuesday night, "The holy war is here."

Here is a brief guide to the global religious war in which we are now all embroiled.

1) What is the holy war?

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O'Reilly used the phrase to describe the fight against ISIS, in which dozens of nations are now participating, including Iran and a number of Arab states.

In his daily "Talking Points" segment, the text accompanying O'Reilly read, "The Holy War Begins: The holy war is here, and unfortunately it seems the President of the United States will be the last one to acknowledge it."

"This is now a so-called holy war between radical jihadists and everybody else including peaceful Muslims," O'Reilly said. "ISIS has murdered thousands of innocent people who follow the teachings of Allah. The terror savages will kill anyone at any time." He later added, "The holy war is here."

2) What does "holy war" mean?

The Encyclopedia Britannica defines "holy war" as "any war fought by divine command or for a religious purpose." It continues, "The concept of holy war is found in the Bible (e.g., the Book of Joshua) and has played a role in many religions. See crusade; jihad."

(Presumably, this definition means that the Encyclopedia Britannica hates Christians, as some US conservatives argued about President Obama when he made a similar point drawing parallels between the Crusades and ISIS.)

The BBC's internal ethics guide offers this guidance for determining whether a war is or is not holy:

However involving God as part of the campaign does not make a war a holy war — for a war to be a holy war, religion has to be the driving force. Holy wars usually have three elements: • the achievement of a religious goal

• authorised by a religious leader

• a spiritual reward for those who take part

The English philosopher Francis Bacon published a 1622 treatise on holy war called "An Advertisement Touching An Holy Warre" that evaluated whether Christian European nations should launch a holy war against the Ottoman Turks. He did not conclusively argue one way or another, but readings tend to view Bacon as seeing war as potentially justified not because the Ottoman Empire was "heathen" but because it was a "cruel tyranny, bathed in the blood of their emperors upon every succession; a heap of vassals and slaves."

3) It kind of sounds like ISIS does think it's fighting a holy war

Indeed it does! As Graeme Wood argues in a feature story for The Atlantic, "What ISIS Really Wants," ISIS and its members are in many ways motivated by their interpretation of Islam and a desire to wage war for their faith, which would seem to meet the definitions of holy war.

Wood's article has spurred a number of debates among Middle East and terrorism analysts, on issues like where ISIS got an interpretation of Islam that is so at odds with the rest of the global Muslim community, or how to account for its non-religious motivations. But basically everyone agrees that ISIS believes it is fighting a holy war.

4) So is Bill O'Reilly saying the US should declare a holy war on ISIS?

Not exactly; O'Reilly says that we're already fighting a holy war whether we like it or not, and we can only win by acknowledging as much.

O'Reilly is very clear that "the holy war is here" already; his point was that it's crucial for Obama to recognize and treat the fight against ISIS as a holy war.

"It is imperative that President Obama stop living in the theoretical, even fictional world," he said of Obama's refusal to call the ISIS fight a holy war.

5) Can we take a music break?

Sure, here is the 1990 Megadeth song "Holy Wars … The Punishment Due."

It includes the lyrics, "Killing for religion / Something I don't understand."

6) What is Bill O'Reilly's advice for winning the holy war?

O'Reilly's strategy is simple: "If the world united against the jihad, it would be defeated."

Unfortunately, rather than elaborating on that point, he goes on to criticize Obama for withdrawing the final US troops from Iraq, earlier in his presidency, after the US and Iraqi governments failed to reach an agreement on a residual force.

7) Why does O'Reilly mean by "unite against the jihad"?

He does not say. But he is very clear that it is outrageous to argue for pursuing any activity whatsoever other than killing members of ISIS.

The US, he said, must discard any tactic that does not involve treating the fight as a holy war.

America is not able to fix economies in third world nations. We are not able to force Islamic despots to respect human rights. We are not able to convince the Iranian mullahs to stop supporting and harboring terrorists and to cease with the nukes. We simply cannot do those things and to waste time throwing them out as possible solutions to dismantling the jihad is insulting and dangerous to the American people. The holy war is here.

8) What spurred all of this?

Bill O'Reilly was responding with outrage to comments by State Department spokesperson Marie Harf, who argued that the US would have to do things other than kill militants to defeat ISIS.

"We're killing a lot of them and we're going to keep killing more of them," Harf said. "But we cannot win this war by killing them. We cannot kill our way out of this war. We need, in the longer term, medium and longer term to go after the root causes that lead people to join these groups."

O'Reilly called the comments "just nonsense." Harf's answer has been characterized as a "gaffe" because it implied that economic factors could contribute to the rise of extremism. O'Reilly and others have rejected this characterization, arguing that it wrongly implies the conflict is about factors beyond religion.

9) It sort of sounds like O'Reilly is endorsing ISIS's narrative

It sure does!

ISIS believes it is fighting a holy war; O'Reilly says that they are correct, and that the United States should engage ISIS on the group's own religious terms. ISIS has shown every indication of wanting to suck the United States into a prolonged ground war; O'Reilly seems to be arguing that we should give them their wish.

O'Reilly also says it is essential that we ignore factors such as mass youth unemployment or authoritarianism that might give rise to extremism or spur young people to join ISIS. Instead, he says, we can only view the conflict within the narrow confines of religious warfare.

This is silly, but it's also dangerous. ISIS insists on seeing the conflict this way out of earnest (if twisted) religious conviction as well as for strategic reasons; framing itself as representing jihadism as in civilizational conflict against the apostates is good for its recruiting and stature. So is antagonizing the US into putting boots on the ground; the 2003 US invasion of Iraq was a boon to jihadist recruitment.

For O'Reilly to say that the US, for example, should have left a residual force in Iraq is a questionable but valid argument that is worth having. For him to argue that Obama must adopt ISIS's worldview and confront the group on its own ideological and strategic terms is alarmingly short-sighted.