This is the most vicious, well-funded and militant terror group we have ever seen.

By Dianne Feinstein
Originally appeared in USA Today

In the ghastly video showing the beheading of American journalist Steven Sotloff, an Islamic State in Iraq and Syria terrorist declares to President Obama, "Our knife will continue to strike the necks of your people."

In the earlier video showing the beheading of James Foley, another U.S. journalist, the ISIS executioner says directly into the camera: "You're no longer fighting an insurgency. We are an Islamic army."

Another recent video, also directed at Americans: "We will drown all of you in blood."

These are ISIS' true colors. Unlike other terrorist groups, ISIS has grown into a sophisticated military force and is estimated to have between 10,000 and 20,000 fighters. The group now firmly controls large swaths of territory in both Syria and Iraq.

The threat ISIS poses cannot be overstated. This is the most vicious, well-funded and militant terrorist organization we have ever seen, and it is very quickly consolidating its power.

This is why I strongly believe the United States must lead an aggressive, international effort to confront and eliminate ISIS, including sustained airstrikes in Iraq and Syria. President Obama has now said that he intends to "degrade and destroy" ISIS. I look forward to hearing the specifics of the president's plan when he speaks on Wednesday. We either confront ISIS now or we'll have to confront it later — when it will be a much stronger enemy.

Weary of war

I recognize the reluctance of many Americans to engage in another war in the Middle East. But it is imperative that every American is fully cognizant of how dangerous and deadly ISIS really is.

First, ISIS is utterly ruthless in its fighting and governance. Last month, a United Nations human rights panel report described the many crimes against humanity ISIS commits on a daily basis. The report notes that "executions in public spaces have become a common spectacle."

In total, the U.N. reports at least 693 child casualties at the hands of ISIS this year. And at least 2,250 women and children are currently detained by the group.

ISIS has killed, enslaved and captured thousands in its efforts at ethnic cleansing, including the Yazidis in Sinjar and the Turkmen in Amerli. Overall, more than a million Iraqis have been displaced.

Sadly, these statistics are just the tip of the iceberg because it is nearly impossible to get a full accounting of the extent of ISIS' brutality on the ground in Iraq and Syria. There are likely thousands of more stories of suffering — of public lashings, rape, abductions, torture and genocidal acts — that have yet to be proved.

Sophisticated operation

Second, ISIS is a sophisticated terrorist army. There is an extremely high level of organization in ISIS operations and ISIS-controlled territory, almost reminiscent of a military dictatorship. ISIS controls extensive resources, military vehicles, heavy weapons and border crossings between Iraq and Syria. It has become a de facto terrorist state. Experts estimate that ISIS now has cash and assets worth $2 billion. ISIS adds as much as $1 million per day through extortion, crime, ransom and even the sale of oil on the black market from the several oil fields it controls.

ISIS is also governing effectively, albeit brutally. It has put in place judicial systems, traffic police and inspectors to ensure sharia law is followed and crime is punished.

Most disturbing is what it seeks to do next. ISIS has its sights set on taking control of additional areas in the Middle East, including Baghdad, where it is already interspersing among the civilian population in Sunni areas.

But ISIS' aspirations don't stop there. In an audio message, the ISIS leader Abu Bakr al-Baghdadi said, "Our last message is to the Americans. Soon we will be in direct confrontation, and the sons of Islam have prepared for such a day. So watch, for we are with you, watching."

I understand that many Americans don't want to become mired in another war. The conflicts in Afghanistan and Iraq have claimed thousands of American lives and cost more than $1 trillion. But Americans need to understand ISIS' degree of viciousness as well as what will happen in the absence of U.S. leadership and action.

If the United States fails to unite and lead the world against ISIS' horrific goals, we could suffer the consequences for decades to come.

Sen. Dianne Feinstein, D-Calif., is chairman of the Senate Intelligence Committee.