CHICAGO -- Democratic Sen. Barack Obama, who is contemplating a run for the presidency, on Monday called for a "gradual and substantial" reduction of U.S. forces from Iraq that would begin in four to six months.

Speaking to the Chicago Council on Global Affairs, Obama envisioned a flexible timetable for withdrawal linked to conditions on the ground in Iraq and based on the advice of U.S. commanders. He also called for intensified efforts to train Iraqi security forces, U.S. aid packages tied to Iraqi progress in reducing sectarian violence and new diplomacy with Syria and Iran.

"I believe that it remains possible to salvage an acceptable outcome to this long and misguided war," he said. "But I have to be honest today, it will not be easy. For the fact is that there are no good options left in this war."

Obama was not in the Senate when President Bush sought and received support from Congress in 2002 to use military force against Saddam Hussein. But he has publicly opposed the war since then.

The results of this month's elections, the Illinois senator said, represented a repudiation of President Bush's policies. But he said the war has also ignited a new sense of isolationism among Americans that is risky in a post-Sept. 11, 2001, world.

"We can't afford to be a country of isolationists in the 21st century," he said, arguing that it is "absolutely vital that we maintain a strong and active foreign policy, relentless in pursuing our enemies and hopeful in promoting our values around the world."

Obama was careful not to set a specific timetable for withdrawal of troops or suggest troop levels.

"We cannot compromise on the safety of our troops, and we should be willing to adjust to realities on the ground," he said.

He proposed redeploying troops to Northern Iraq and to other countries in the region. He recommended boosting troop strength in Afghanistan, "where our lack of focus and commitment of resources has led to an increasing deterioration of the security situation there."

"For only through this phase redeployment can we send a clear message to the Iraqi factions that the United States is not going to hold together this country indefinitely _ that it will be up to them to form a viable government that can effectively run and secure Iraq," he said.

Obama rejected proposals to add more troops to Iraq, an idea advanced by Sen. John McCain, R-Ariz., saying that without Iraqi cooperation "we would only be putting more of our soldiers in the crossfire of a civil war."

Following the speech, Obama rejected the notion of reinstating the military draft, an idea put forth by Rep. Charles Rangel, D-N.Y. Obama said a volunteer military is adequate when the country has a national security strategy that makes sense.

"What I don't want ... is a situation in which we have bad strategies and we institute a draft simply to throw more bodies at a bad strategy. That is not going to work," he said.

Obama is one of several Democrats considering a run for the White House in 2008, which promises to be one of the most wide-open campaigns in decades. In recent polling, his numbers rival front-runner Sen. Hillary Rodham Clinton, D-N.Y., but questions abound regarding his experience _ he has served less than two years in the Senate. He said Monday he would decide in the next few months.

The first-term senator said the Iraq war has underscored one lesson: "We should be more modest in our belief that we can impose democracy on a country through military force."