All U.S. combat forces will be out of Iraq by the end of August 2010, but many of those troops will stay in the war zone throughout 2009, President Barack Obama said on Friday.

"The most important decisions that have to be made about Iraq's future must now be made by Iraqis," Obama said before hundreds of Marines and officers at the sprawling Camp Lejeune, N.C., base, which is deploying thousands of troops to the other war front -- in Afghanistan.

Obama's much-anticipated Iraq strategy is a marked departure from the previous Bush administration's. Former President George W. Bush, and Obama's Republican opponent in the general election, Sen. John McCain, opposed setting specific dates for withdrawing U.S. troops from the war-torn country.

But the certain date of what he called "responsible" troop withdrawal was a centerpiece of Obama's successful presidential campaign. He told the gathered Marines that he was moving forward with his plans after careful deliberations with Secretary of Defense Robert Gates and commanders on the ground in Iraq. And he offered caution about the way forward.

Obama applauded the military for its role in an improved situation in Iraq, where violence is down significantly in Baghdad and most of Iraq and U.S. military deaths have plunged. But he said, "We cannot rid Iraq of all who oppose America or sympathize with our adversaries.

"We cannot police Iraq's streets until they are completely safe, nor stay until Iraq's union is perfected. We cannot sustain indefinitely a commitment that has put a strain on our military, and will cost the American people nearly a trillion dollars."

At the same time, Obama said, "Every nation and every group must know, whether you wish America good or ill, that the end of the war in Iraq will enable a new era of American leadership and engagement in the Middle East," he said. "This does not lessen our commitment. We are going to be enhancing that commitment to bring about a better day in that region, and that era has just begun."

Of the roughly 100,000 U.S. combat troops that Obama proposes to be pulled out of Iraq over the next 18 months, most will remain in the war zone through at least the end of this year to ensure national elections there go smoothly.

Even with the drawdown, a U.S. force of 35,000 to 50,000 U.S. troops will stay in Iraq under a new mission of training, civilian protection and counterterrorism.

There are currently 142,000 U.S. troops in Iraq.

The war has cost $650 billion since 2003. About 4,250 U.S. troops have died and more than 31,000 have been wounded since the war began.

Obama Expands National Security Team

Obama also is making some national security changes to reflect the realities of modern threats by expanding the membership and reach of his top national security team.

New concerns involve the nation's vast cyber networks, the delivery of energy, and the effects of the economic downturn on fragile governments, the administration said.

The Associated Press has obtained a copy of the presidential directive detailing the shift, which is the first penned by Obama.

Under the new structure, representatives from a broad range of federal agencies will be invited to meetings when the issue affects their area of expertise.

Two other key reviews look at cybersecurity and at shifting some responsibilities of the Homeland Security Council to the National Security Council.