Withdrawal of the American forces in Iraq and support from the Republicans

First background information:
President Barack Obama, has finally anounced the long waited declaration of withdrawal from Iraq by many Democrats and Republicans. This announcement was especially important as it was the main difference between Mr.Obama and Mr.McCain in the elections and by supporting the withdrawal of the american forces, Mr.Obama didn't only earn the respect of many all around the globe and earned many voters that complained from the ongoing wars by destroying the deep distinction between the Republican voters and the Democrat voters to some extent.
The plan as announced will be terminated at december 2011 with the full withdrawal of all american forces in Iraq. However in the first stage the plan is the withdrawal of the american forces in August 2010, leaving a 50000-30000 soldiers behind to be evacuated a 2011 out of 142000 soldiers.
From my point of view, the plan seems to be promising with only one concern. The founders of the plan explains the necessasity of a two stage point by saying that it ensures the security of the forces that will be withdrawn at the second stage. The concern shared by many democrats and me as well is due to the huge amount of 50000 soldiers that will be left at the region until December 2011. I don't know the details of the concerns of the democrats, however my concern is that the withdrawal date of those 50000 soldiers can be easily postponed due to any event or action not necessarily from Iraq but any third party such as Iran. No matter who is the president of USA, the policies of USA isn't altered to a significant degree as they are the commpn product of not a single product but a series of governments. Thus it is very probable that those 50000 soldiers can be used in a new military operation or postponed to stay to a further date.