Franks, who planned and commanded the American-led wars in Iraq and Afghanistan, said the United States was in a crucial struggle whose outcome would "craft the next 200 years" of American history.
"Two wars were won," he said. "Now two peaces must be won."
The retired general was the keynote speaker at the annual Alfred E. Smith Foundation Memorial dinner at The Waldorf-Astoria hotel. More than 800 people paid $800-a-plate for the dinner, which raises money for local Roman Catholic hospitals.
The event is named for the former New York governor who was the unsuccessful 1928 Democratic presidential candidate and the first Catholic to run for president.
Speeches at the dinner are traditionally humorous, and for the most part, Franks' address was lighthearted. Last year, before the U.S.-led war in Iraq, Secretary of State Colin Powell wisecracked about Iraqi President Saddam Hussein, mocking the strongman's nearly unanimous election victories. Vice President Dick Cheney spoke at the dinner in 2001.
This time, Franks cracked that Gov. Smith's great-grandson, Alfred E. Smith IV, had two other people in mind as keynote speakers when he planned this year's dinner.
"Al wanted a man who was an expert in American weapons systems, but unfortunately, Saddam Hussein and Usama bin Laden were not available," Franks said, drawing laughter.
Franks did strike a serious tone, however, saying the war against terrorism was necessary, regardless of the costs. He said the Sept. 11 attacks had changed the way Americans view the world.
"We saw a tragic act. On that day, we learned ... of our own vulnerability," he said.
The following day, "we learned something else," Franks continued, a bit more jocular. "We started to see American attitude. A superpower on steroids."
Franks resigned this summer. He was replaced by Army Gen. John Abizaid, who formerly oversaw military operations from the Horn of Africa to Central Asia.
In his speech, the Franks joked about his humble upbringings in Texas.
"You take a good Ole boy out of Midland, Texas, and put him on the dais in this by-God Waldorf Astoria, in New York City, amongst politicians, preachers and rich folks," he said. "By God, ain't this country great?"