WASHINGTON -- The political battles over U.S. war policy shifted toward Afghanistan yesterday, as President Bush announced an influx of troops there, while the two presidential candidates, Sens. John McCain and Barack Obama, attacked each other's commitment to defeating terrorists.

In describing perhaps his last major military deployments as commander-in-chief, Mr. Bush announced that he would accept Pentagon recommendations to remove about 8,000 U.S. troops from Iraq by early next year. But the president also said he would send nearly 5,000 troops to Afghanistan, which he characterized as an increasingly important front in the battle against al-Qaida.

"Al-Qaida's leaders have repeatedly declared that Iraq is the central front of their war with America, but it is not the only front," Mr. Bush said at the National Defense University in Washington. "As al-Qaida faces increased pressure in Iraq, the terrorists are stepping up their efforts on the front where this struggle first began -- the nation of Afghanistan."

The announcement underscored the reemergence of Afghanistan in the debates over U.S. national security and illustrated how, nearly seven years after the Sept. 11 attacks, Democrats and Republicans remain at odds over the best strategy for fighting al-Qaida and other Islamist extremists.

Mr. Obama, a Democrat from Illinois who has long called for more U.S. troops in Afghanistan, said at an Ohio news conference yesterday that Mr. Bush "is moving in the direction of the policy that I have advocated for years." But he said the plan also "comes up short" because "it is not enough troops and not enough resources, with not enough urgency."

"What President Bush and Senator McCain don't understand is that the central front in the war on terror is not in Iraq, and it never was," Mr. Obama said. "The central front is in Afghanistan and Pakistan, where the terrorists who hit us on 9/11 are still plotting attacks seven years later."

By contrast, Mr. McCain, a Republican from Arizona, praised Mr. Bush's announcement of Iraq withdrawals as a demonstration of "what success in our efforts there can look like," and alleged that Mr. Obama "believes we must lose in Iraq to win in Afghanistan." He criticized his Democratic rival for opposing an increase in troops in Iraq last year that the administration credits for reducing violence.

"Senator Obama's comments today demonstrate again his commitment to retreating from Iraq no matter what the cost," Mr. McCain said in a statement. "His focus is on withdrawal -- not on victory."

Mr. Bush spent nearly half of his speech focusing on the worsening conflict in Afghanistan, where the increasing death toll among foreign troops has surpassed those in Iraq in recent months. The new deployments represent a 15 percent increase in U.S. military personnel for Afghanistan, and administration officials say the groundwork is being laid for more troops in the future.

Senior leaders at the Pentagon have said for months that they need additional U.S. troops to combat growing violence in Afghanistan, but ongoing military commitments in Iraq made such a move impossible until now.

"Changed circumstances means changed resources, changed commitments," the administration official said.

Also yesterday, the U.S. Central Command announced that Brig. Gen. Michael W. Callan, former commander of the Air Force Special Operations Forces, will head a new military investigation of a U.S. airstrike last month in Afghanistan that U.N. and Afghan officials say killed 90 civilians.

Gen. Callan and a small team, including a military legal official and officers with experience in Afghanistan, will "consider new information that has become available since the completion of the initial investigation," a statement said.

The initial military review of the Aug. 21 incident in the town of Azizabad, near the western Afghan city of Herat, found that five to seven civilians had been killed, along with 30 insurgents.

In his remarks, Mr. Bush said U.S. successes against al-Qaida and other extremists in Iraq have played a role in the growing violence in Afghanistan.

"The Taliban and al-Qaida will not be allowed to return to power," Mr. Bush said.

Brian Jenkins, a terrorism expert at the Rand Corp., said Mr. Bush's characterization of Afghanistan is oversimplified and ignores evidence that the Taliban and its al-Qaida allies regained strength there because the U.S. military was focused on Iraq.