That is not to say that when Congress authorizes war, it gives the president a blank check. Just as Congress may transgress the constitutional boundary, so too may the president—and indeed, the threat to proper allocation of power comes most frequently from the executive's claim to broad emergency powers. Congress can define the laws of war. And so, the Bush administration's views to the contrary notwithstanding, it can order the president not to torture or mistreat detainees, and not to engage in certain kinds of espionage.
Just as important, Congress may specify the geographical scope of the war, limiting it to Iraq and not Iran, or Vietnam and not Laos, as it did (rather ineffectually) in the 1970s. Similarly, Congress may define the nature of a war, authorizing for example an air war but not a ground war, as it did in Kosovo, or a blockade on ships entering port but not on ships leaving port, as it did in the late 18th century during a limited sea conflict with France. Congress could tell the president that a given war should be conventional, not nuclear.
The constitutional test, in the spirit of Justice Jackson, should be whether Congress is defining the conflict in a way that makes realistic sense in the light of how modern conflict is actually fought and understood by modern combatants. It would be practically impossible for forces in Iraq to fight only al-Qaida and no one else, since al-Qaida wears no uniforms and cannot in combat be distinguished from other elements of the Iraqi insurgency. So, a law ordering our troops to fight al-Qaida and no one else in Iraq would be unconstitutionally vague, as well as unrealistic.
Finally, through the power of the purse, Congress inevitably determines the resources the president has to fight the war. If Congress has only paid for a certain number of battleships or up-armored Humvees or, indeed, troops, these are what the president will have at his disposal. But it does not necessarily follow from this that Congress can tag certain weapons for service only in one place and not in another once we are at war. Or that it can direct money allocated for troops to be used only if the troops are stateside.
In exceptional cases, Congress has in the past limited troop numbers in a particular theater, as when the Vietnam conflict came to a close. But this occurred only as part of a withdrawal of all troops, and only after a final peace treaty had been signed. In a January letter to Congress, a group of distinguished constitutional scholars argued that Congress could permissibly place a "ceiling" on the number of troops assigned to Iraq.
But in practice, specifying the number of troops comes perilously close to the kind of micromanagement by Congress that the same scholars specifically repudiated as unconstitutional in their letter.
It might conceivably be argued that adding troops in Iraq changes the nature of the war and so must trigger a new authorization of war. But the realities of this war contradict this imaginative interpretation. Troop levels have fluctuated up and down by more than 20,000 troops with some regularity since March 2003, depending upon the strategy and tactics chosen by the president and the schedule of deployment and redeployment. These fluctuations were part of a (failed) attempt to win the war, not some fundamental change in its nature or geographical scope.
Preventing Congress from capping troop levels in Iraq suggests that the president could tomorrow reassign troops from Afghanistan to Iraq—or, indeed, from Fort Bragg to Iraq—and Congress could do nothing about it. But there is nothing especially strange about this. The president does just this every day when he decides which troops will be assigned where (through the secretary of defense). What would be strange would be Congress telling the president that a particular unit must serve in Iraq or Afghanistan, or must be ordered home.
Indeed, as far as we are aware, at no time in our history has Congress claimed the right to exercise any war power beyond the following: 1) to declare, undeclare, and provide funds for war; 2) to define the field of battle and the nature of the conflict; 3) to enforce law-of-war limitations on the conduct of warfare; 4) and to demand that the president report emergency military actions to Congress for its approval within a fixed period of time.