The generals have a better understanding of troop movements

Ronald Cass:

Georges Clemenceau famously observed that "war is too important to be left to the generals." This well-known quotation isn't exactly what he said, but it comes close and sounds better than his actual words. Unfortunately, in war -- unlike reporting, editorializing or delivering political speeches -- getting things exactly right matters a lot.
Congressional Democrats' efforts to put their own stamp on the war in Iraq responds to a political instinct to do what sounds good rather than the military imperative of doing what is best calculated to work. House Democrats, under John Murtha's guidance, pushed through a bill that threatens to do a great deal of damage to our war effort and to our security. Their colleagues in the Senate followed suit. Both efforts show Congress at its worst -- giving far more weight to pork, pomp, and posturing than to our safety and constitutional traditions.
The essence of Clemenceau's epigram was that, at bottom, war is not only about strategy and tactics. It is also about political goals and national values that transcend the special competence of generals. But not everything is for the politicians, much less for all politicians.
Some parts of the war effort -- any war effort -- lie squarely in Congress' hands. Basic funding decisions, including funding for our military, are given to Congress by the Constitution. So is the authority to declare war, to authorize use of military force. Congress exercises these constitutional powers through its lawmaking function.
The constitutional and historical assignment of those functions to Congress reflects their compatibility with the nature of congressional action. They fit comfortably with public debate, deliberation and compromise -- hallmarks of a large body of political representatives with differing constituencies, interests and objectives.
Not every function is equally suited to collective action by 535 individuals with diverse interests, strong views, and far greater facility at public speaking than at secrecy. You wouldn't ask such a group to write a sonnet, nor to direct a war.
The conduct of war requires direction from a single authority capable of making decisions with speed, secrecy and flexibility. Changing circumstances on the ground don't permit the luxury of extended debate. Military strategy doesn't benefit from accommodation of different judgments informed by competing interests.
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Democrats in Congress, regrettably, seem more intent on scoring political points than on helping make our military efforts effective....
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Legislators do not make a good war counsel. They do not comprehend the information needed to make a decision. Within the military there will be a counsel of leaders who will come up with a plan to meet the objectives they have been given by the national command authority, i.e. the President.

They will weigh the options and tell the President whether his objectives are achievable. If it cannot they will say so, for their is no incentive on their part to engage in a failed mission. They more than members of Congress know whether a mission is achievable.

What Congress can do is decide that the cost of such a mission is not something they are willing to fund. However the Democrats are not willing to take responsibility for such a decision so they conspire to make the mission impossible.