One Army brigade and one Marine brigade will be sent to Afghanistan as part of the escalation, which represents a 50 percent increase in the current size of the U.S. force there. Some of the troops had already been ordered to Iraq, and will be diverted. As NBC's First Read blog observes, that move "permit[s] the White House to claim in this one announcement that President Obama is making good on two campaign promises -- withdrawing forces from Iraq and sending additional forces to Afghanistan."
Excerpts from Obama's statement:
There is no more solemn duty as President than the decision to deploy our armed forces into harm’s way. I do it today mindful that the situation in Afghanistan and Pakistan demands urgent attention and swift action. The Taliban is resurgent in Afghanistan, and al Qaeda supports the insurgency and threatens America from its safe-haven along the Pakistani border...
This increase is necessary to stabilize a deteriorating situation in Afghanistan, which has not received the strategic attention, direction and resources it urgently requires. That is why I ordered a review of our policy upon taking office, so we have a comprehensive strategy and the necessary resources to meet clear and achievable objectives in Afghanistan and the region. This troop increase does not pre-determine the outcome of that strategic review. Instead, it will further enable our team to put together a comprehensive strategy that will employ all elements of our national power to fulfill achievable goals in Afghanistan. As we develop our new strategic goals, we will do so in concert with our friends and allies as together we seek the resources necessary to succeed.