Joe Biden, the US vice-president, said last month that the US drawdown was already "well under way".
'Bombs and bullets'
The presence of US forces in Iraq is governed by the so-called "status of forces agreement," negotiated during the Bush administration.
"Because of the sacrifices of our troops and their Iraqi partners, violence in Iraq continues to be near the lowest it's been in years"
It calls for the US combat mission to end by September 1, and for all US troops to leave Iraq by the end of 2011.
Obama cautioned that "those with bombs and bullets ... will try to stop Iraq's progress".
He praised recent security gains in the country and said Iraqi security forces have "already taken the lead for security" in many parts of the country.
"Because of the sacrifices of our troops and their Iraqi partners, violence in Iraq continues to be near the lowest it's been in years," he said.
But violence in Iraq has been creeping upward in recent weeks. Statistics released on Sunday from the Iraqi defence, health and interior ministries showed that 535 people were killed in July, making it the deadliest month in Iraq since May 2008.
The US military rejected those figures, saying that 222 people were killed in July.
'On the offensive'
In Monday's speech, Obama also defended his decision to send 30,000 additional troops to Afghanistan.
"If Afghanistan were to be engulfed by an even wider insurgency, al-Qaeda and its terrorist affiliates would have even more space to plan their next attack," Obama said, repeating language he has used on a number of occasions.
Obama also spoke optimistically about his strategy in Afghanistan.
He said that Nato forces are "on the offensive against the Taliban," and that the Afghan government has "taken concrete steps to foster development".
Obama also praised Pakistan's efforts to "take the fight to violent extremists within its borders".