President Bush's speech may be scheduled for tonight, but the troop surge in Iraq is already under way.
ABC News has learned that the "surge" Bush is expected to announce in a prime time speech tonight has already begun. Ninety advance troops from the 82nd Airborne Division arrived in Baghdad Wednesday.
An additional battalion of roughly 800 troops from the same division are expected to arrive in Baghdad Thursday. Eighty percent of the sectarian violence occurs within a 30-mile radius of Baghdad, so that is where most of the additional troops will be concentrated.
It is the first small wave of troops in a new White House strategy that is expected to put more than 20,000 additional U.S. troops on the ground in Iraq and likely require new call-ups of the National Guard.
The president is expected to deliver his announcement about the troop increase, a plan that has already met with stiff criticism from many members of Congress, in a speech tonight from the White House.
The arrival of additional forces in Iraq comes a day after leading Democrats said they would back legislation that would block funding to pay for additional military forces.
A spokeswoman for Sen. Edward Kennedy, D-Mass., who, Tuesday, urgently called for Congress to vote on -- and reject -- the proposed surge, told ABC News that the arrival of additional soldiers "underscores Sen. Kennedy's point that Congress must act immediately."
Senate Majority Leader Harry Reid, D-N.M., also responded to the report, calling the troops' arrival "deeply disappointing."
Reid has said that Americans don't want to see additional forces in Iraq, and that he has been considering plans offered by his congressional colleagues.
Military commanders say there is more to this plan than boots on the ground -- it's also how the troops will be used.
The idea is for U.S. and Iraqi forces to become a more integral part of Baghdad neighborhoods such as Dora, which was secured in August 2006 only to see violence spike when U.S. forces left.
Under the new plan, the city of Baghdad will be divided into nine separate sections at the request of Iraqis, who want one army and police battalion devoted to each section.
The additional U.S. troops being sent to Baghdad will be divided among the nine sections of the city, nearly doubling U.S. combat power in the region.
In a switch from the current course of action, these U.S. forces will be housed in the very neighborhoods they patrol. Military planners tell ABC News there will eventually be about 30 mini bases, called joint security stations, scattered around Baghdad, housing both U.S. and Iraqi troops.
The plan also includes an emphasis on performance from the Iraqis. White House officials said they have put tremendous pressure on Iraqi Prime Minister Nouri al-Maliki to crack down on Shiite militias, especially radical cleric Moqtada al-Sadr, who is thought to be responsible for most of the sectarian violence.
A senior White House official said that Maliki told the president, "I swear to God, I'm not going to let Sadr run this country."
Maliki must also provide Iraqi troops on schedule and give Sunnis a larger role in the government.
Tonight, the president is expected to say that he's made it clear to the prime minister and Iraq's other leaders that America's commitment is not open-ended, that now is the time to act.
The president is also expected to announce tonight the deployment of a second aircraft carrier -- perhaps the USS Stennis -- to the Persian Gulf, as part of a regularly scheduled deployment. Centcom Commander Gen. John Abizaid has requested a second carrier because of Iran and other threats in the region.
The ship will leave next weekend on its regularly scheduled departure date, but will proceed to the Gulf instead of its original deployment to the Pacific.
Commanders believe the new approach will make U.S. forces better positioned to combat sectarian violence, but they acknowledge this approach is riskier and will likely mean more U.S. casualties in the short-run.
In al-Anbar Province, an additional 4,000 Marines will focus on fighting al Qaeda to try to take advantage of what U.S. commanders say is a new willingness of some Sunni tribal leaders to cooperate in the fight against the international terrorist organization.
The increase in U.S. forces will be completed within 90 to 120 days.
ABC News has also learned that National Guard troops who have seen combat in Iraq and Afghanistan will likely be getting some unwelcome news: They may have to go back to Iraq later this year. National Guard combat teams would be sent to Iraq as the next wave of the surge, unless the first wave succeeds in reducing the violence.
Commanders here caution it will take several months to fully implement the plan and maybe even longer to see results. As one senior military official here said Wednesday, "We don't know if this will work, but we do know the old way was failing."
ABC News' Jake Tapper and Martha Raddatz contributed to this report.