Zinni said in a speech Thursday he expects American troops to be in Iraq for five to 10 years, facing a "witch's brew" of insurgents, criminals and "Al Qaeda (search) wannabes."
The Bush administration has said it intends to keep troops in Iraq for as long as it takes the country to achieve stability, but it has not been more specific.
Zinni served as commander in chief of the U.S. Central Command from 1997 to 2000 and as a special Middle East envoy from 2001 to 2003. He spoke during a lecture hosted by the Foreign Policy Association at Hunter College.
The Iraqi elections scheduled for January will be the next crucial moment in that country, Zinni said.
"This has to work," he said, referring to the elections. "It may not be perfect, it may not be that every province can fully participate, but it has to go down in January, and it has to have some structure to it."
He warned that insurgents would do everything they could to disrupt the elections and predicted civil war in Iraq if they are successful.
"It would be a fractured Iraq," he said. "It would be a cesspool of the kind of things that would threaten the region."
Zinni was an early critic of the plan to invade Iraq, calling it a distraction from the war on terrorism and harmful to fostering peace in the Middle East. Since retiring last year he has stepped up his remarks and, with author Tom Clancy, recently published a book, "Battle Ready," which looks back at his military career and includes strong words against the war.
Echoing recent remarks by L. Paul Bremer, the former top U.S. administrator in Iraq, Zinni said Thursday that the United States never had enough troops on the ground and, "I think we are still paying the price."