On his Fox News show Monday night, Bill O'Reilly suggested using mercenaries to fight the Islamic State (ISIS) instead of U.S. ground forces, which President Obama has repeatedly vowed not to deploy.

The use of mercenaries is as old as warfare itself. Alexander the Great used them. King George III hired German mercenaries to fight for the British in the American Revolution. Today mercenaries go by other names, like “contractor."

The president is to be commended for assembling a coalition that includes Arab states, but why are our European allies not part of it? Britain hasn't sent planes, though British citizens have been killed by jihadists. An ISIS splinter group in Algeria claims to be holding a French national hostage and vows to kill him unless France halts its attacks on ISIS positions in Iraq, but France isn't joining the president's coalition to hit ISIS targets in Syria.

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Under O'Reilly's scenario, mercenaries would be well paid and live under American rules of war and the Geneva Convention. They would solve a political problem for President Obama and his liberal base that wishes not to be a part of any Mideast conflict and, more importantly, it might be more effective in achieving the president's goal of "degrading and destroying" ISIS far more than airstrikes alone.

Daniel Trombly and Yasir Abbas are analysts with Caerus Associates, a research and strategy firm based in Washington. In an article for The Daily Beast, they get to the heart of what's wrong with a bombing-only strategy:

"The more familiar but difficult medium- to long-term task of degrading ISIS's operational leaders and eventually its high leadership will take well-disciplined, organized ground forces to push out ISIS guerrillas entrenched among population centers where airpower cannot remove them. It also will take major improvements of the quality of intelligence, especially in Syria, where U.S. relationships are least developed and its forces are most unfamiliar with the local environment. Strategically, airpower will play a supporting role to efforts to organize and coordinate ISIS's rivals on the ground to retake territory permanently and contain it as a regional threat. Even as ISIS's rapid offensive gains have proved limited and vulnerable to modern air attack, the group's capabilities as a defensive and clandestine guerrilla force -- and its decentralized military structure -- will deny foreign airpower a rapid or comprehensive victory in the long-term effort for its defeat."

What's needed most is a change in American and Western thinking. This war against a constantly shifting force of what is, despite denials by the president and Secretary of State John Kerry, a religious-political virus, is likely to last years, perhaps decades.

When American leaders stop trying to turn the motivations of fanatics into something other than what they are, only then will we start treating this war for what it is. All of the non sequiturs about Islam being a "peaceful religion" that has been "hijacked" by extremists is meaningless if members of the "peaceful religion" don't rise up and defeat those fanatics they claim have misrepresented their faith.

Though Arab states joining the coalition is a start, the fact that these nations are not yet providing ground forces to defeat these "apostates" and "heretics" tells us something. Are they afraid of ISIS and other Islamist groups? Do they share some of their goals?

Either way, if Western civilization and its values of tolerance, freedom and religious pluralism are to survive, we have to make sure that ISIS does not. There can be no co-existence between good and evil. If good doesn't triumph, evil will.

Mercenaries might be an effective tool in defeating evil.

Cal Thomas is America's most widely syndicated op-ed columnist. He joined Fox News Channel in 1997 as a political contributor. His latest book is "What Works: Common Sense Solutions for a Stronger America" is available in bookstores now. Readers may email Cal Thomas at tcaeditors@tribune.com.