So Bush is announcing the "Surge" of troops on Wednesday. Congress is already lining up, for and against. (More against)I'm guessing this is going to land with a thud, in both polls and Congress. But it will happen anyway, given the system. Bush is Commander in Chief, which means the military reports to him. So if he says send more troops, more troops are going to go.
Now Congress and the people who elect them don't want this "surge" but their options are limited. There is Congress's "power of the purse" but cutting off funds for the "surge" or the war in general is politically tricky and may not work. As Barney Franks points out, Bush can simply move money about the Pentagon to work around any budget blocks Congress puts in his way. Which makes stopping him hard, short of defunding the entier Pentagon.
So we are stuck.