Asked what he's learned about ISIS that he didn't know a week ago, Carter said, "Well, I wouldn't say that anything that happened over the last few days is surprising to me. It shouldn't really be surprising to anyone."
"This is an enemy that needs to be defeated, will be defeated, stands for the opposite of everything we stand for as civilized people," he added, speaking at the Wall Street Journal Chief Executive Officer Council annual meeting.
Carter expressed concern about ISIS' growing external influence -- both in the social media sphere and in their recently demonstrated ability to plot attacks outside Iraq and Syria.
"The metastasis of this has to be taken care of," said Carter, "as well as the parent tumor, which is in Syria and Iraq."
There is "no question," Carter also noted, that ISIS is getting better at avoiding detection by intelligence services.
But Carter suggested a Paris-style attack would be less likely in the United States.
"We've been concerned about it since last summer because they [ISIS] say they have the aspiration to come here," Carter said, but added, "their capability is not what it is in Europe. There isn't as much ease, geographically, and movement of people."
Carter also suggested there aren't the same kinds of communities in the U.S. as there are in Europe, where terrorist sympathies are prevalent.
"The more immediate danger we face is more of a lone wolf kind," he added, citing the attack in Chattanooga, TN as an example of that threat.
In his remarks, Carter highlighted some of the successes the U.S. has had against ISIS in the Middle East, specifically recent airstrikes against the British militant "Jihadi John," the leader of ISIS' affiliate in Libya, and oil infrastructure the group has derived revenue from.
Weighing in on Russian involvement in Syria, Carter said Putin's government has "made a mistake," and that they are "doomed to fail" as long as they continue to prop up Syrian president Bashar al Assad.
"They need to see that they're on the wrong side of things," said Carter, "get on the right side, because they could be persuasive with Assad."