Obama's decision greatly expands the scope of the U.S. campaign and the geographic distribution of American forces, some of whom will head into Iraq's fiercely contested western Anbar province for the first time to act as advisors.
It also raises the stakes in Obama's first interactions with Congress after his Democratic Party was thumped by Republicans in mid-term elections this week. The White House said it would ask Congress for $1.6 billion for a new "Iraq Train and Equip Fund."
Pentagon spokesman Rear Admiral John Kirby said those funds would need to be approved before the first additional forces headed to Iraq, something one official speculated could happen in just weeks.
Kirby told a Pentagon news briefing that Defense Secretary Chuck Hagel was urging Congress to approve the funding as soon as possible. It is part of a larger $5.6 billion supplemental spending request.
Alarmed by the advance of Islamic State militants across Iraq, Obama began sending non-combatant troops back to Iraq in the summer for the first time since he withdrew U.S. forces from the country in 2011.
Officials denied the troop buildup amounted to "mission creep" and said it was justified partly because of Iraqi Shi'ite efforts to reach out to Sunni tribesmen after the election of Shi'ite Prime Minister Haider al-Abadi.
AUTHORITY FOR 3,100 U.S. TROOPS
One Obama administration official, speaking on condition of anonymity, pointed to an Iraqi plan to "organize and equip 5,000 tribesmen in Anbar."
"This is now being openly discussed in Iraq and it's starting to happen," the official said.
About 1,400 U.S. troops are now on the ground, just below the previous limit of 1,600 troops. The new authorization gives the U.S. military the ability to deploy up to 3,100 troops.
Kirby said many of the additional American troops would be dedicated to securing bases where training and advising would take place, but he cautioned that American troops still face risks.
"We already had a couple of military deaths associated with this conflict ... Nothing we do is without risk," he said.
The Pentagon said it planned to establish several sites across the country to train nine Iraqi army brigades and three brigades of Kurdish Peshmerga fighters. They would be set up in northern, western and southern Iraq.
The U.S. military would also establish "advise and assist" operations centers, adding to similar centers in existence in Baghdad and Arbil.
FIGHTING IN ANBAR PROVINCE
Officials said one location to which military advisors would soon travel was western Anbar province, bordering Syria, where Islamic State fighters are on the offensive.
Iraq's main military divisions in Anbar have been hit hard. At least 6,000 Iraqi soldiers were killed through June and double that number have deserted, according to medical and diplomatic sources.
The announcement of the force expansion was made on the same day Obama met with members of Congress at the White House and updated them on the fight against Islamic State in Iraq and in Syria.
Obama's Iraq campaign has been criticized, particularly by some Republicans concerned about his determination to limit the U.S. role to air strikes and advising and training missions far from the front lines.
U.S. Representative Buck McKeon, a California Republican, said in a statement, "I would urge the President to reconsider his strategy and clearly explain how this additional funding supports a new direction. Such clarity is more likely to find swift Congressional approval."