The plan would require somewhere between 6,000 and 10,000 U.S. boots on the ground. "Not an insignificant number," he admitted, compared to the 5,000 American soldiers who are in the war-torn area today.

Stavridis detailed a four-step plan for defeating the terrorist group on CNBC's " Squawk Box ," where he made a case for a significant military push into parts of Iraq and Syria .

It's time for America to get aggressive with ISIS, retired Adm. James Stavridis, former NATO supreme allied commander and current Hillary Clinton advisor, said Tuesday.

The retired admiral and current dean of the Fletcher School of Law and Diplomacy at Tufts University also spoke to what he sees as the differences between Clinton's and Donald Trump's campaigns.

"A Trump administration, as you look at their approach to foreign policy, is more about building walls, about creating distance, [and] about heightening security," Stavridis said.

"A Clinton campaign," he continued, "is about building bridges, not walls. It's about connections, it's about international, interagency, private-public connections to try and create security."

"The right answer is probably a bit of both," he acknowledged, "but I would say, broadly speaking, we need more building of bridges than we do building of walls."

Stavridis had been on Clinton's shortlist for a running mate. The former secretary of state ended up choosing Virginia Sen. Tim Kaine, who on Tuesday night faces Trump's No. 2, Indiana Gov. Mike Pence, in the only vice presidential debate.