Americans, a litigious people, believe that rules for coping with messy reality can be written in tidy legal language. This belief will be tested by the debate that will resume when Congress returns from a recess it should not have taken, with a war to authorize. The debate concerns an Authorization for Use of Military Force (AUMF) against the Islamic State and also against …

Well. The debate’s difficulty defines its urgency: It is hard to say precisely against what (does the Islamic State’s name make it a state?), and therefore where, force should be authorized. This debate will demonstrate the limits of legalistic precision in war. Which is why, once war begins, limiting presidential war-making power is like lassoing a locomotive with a cobweb. So, this overdue debate – six months and approximately 2,000 U.S. airstrikes into the war on the Islamic State – properly should preoccupy Congress for weeks.

Consider a former authorization of force: “Congress approves and supports the determination of the president … to take all necessary measures to repel any armed attack against the forces of the United States and to prevent further aggression.” The previous aggression was an attack, a few days earlier in August 1964, on USS Maddox, a destroyer, in the Gulf of Tonkin. Of the 58,286 names on the Vietnam Veterans Memorial, 57,988 are of Americans killed after passage of the Gulf of Tonkin Resolution, which was the closest Congress came to declaring war.

The United States last declared war on June 5, 1942, against Hungary, Romania and Bulgaria. Congress can perhaps be said to have “authorized” America’s first conflict after World War II by promptly funding it when North Korean forces crossed the 38th parallel in June 1950. But merely post facto consent leaves presidents as no president should be: completely unfettered. And vulnerable: It is bewildering that presidents often are reluctant to bring a collaborative Congress along when they step into the fog of war, where some things always go wrong.

The Constitution’s Framers weighed their words carefully. Enumerating Congress’ powers, they said it could “raise and support armies” and “provide and maintain a navy.” An army, the Framers’ recent experience told them, could be summoned into existence – raised – more easily than could a navy, which takes more time to build and is more expense to maintain. The Framers gave Congress power to “declare” war and gave the president power to conduct war. This distinction primarily acknowledged the possible necessity for presidents to act unilaterally to repel sudden attacks when Congress was frequently dispersed and impossible to reconvene quickly.

The Framers never, however, contemplated the conundrum today confronting presidents and Congresses who must respond to war waged by metastasizing non-state entities like the Islamic State. In 1802, just 13 years after ratification of the Constitution, Congress authorized President Thomas Jefferson to send “armed vessels” against pirates conducting “predatory warfare against the United States.” But Congress located responsibility for the attacks to a state-like entity, “the regency of Tripoli.”

You might think that no one would argue that any president may legitimately wage war anywhere he chooses, against anyone he chooses, with any means he chooses, for any period he chooses, and that congressional authorization is an optional and perhaps anachronistic filigree on the national security process. That is, however, essentially the thinking of a congressional faction that can be called the Article II Caucus. It believes that the president’s Article II power to wage war cannot be prudently, or perhaps even constitutionally, circumscribed with geographic, time or other limits.

Those who believe this object to President Obama’s proposed AUMF as too restrictive. It would forbid “enduring offensive ground operations.” We have Ira Gershwin’s word for it: The Rockies may crumble and Gibraltar may tumble. Almost nothing “endures,” which is an elastic, non-limiting term.

Actually, Obama’s proposal is extraordinarily permissive because it authorizes force against the Islamic State and “associated forces.” What defines association? Operational coordination? Or just shared doctrines and goals? If the latter, is Boko Haram “associated”? If so, would Obama’s AUMF authorize intervention in Nigeria and anywhere else in sub-Saharan Africa afflicted with groups “associated” by doctrines and goals with Boko Haram? Because the Islamic State has the charisma conferred by momentum and audacity, many groups are claiming not just kinship but association.

Congress, with distinctions to draw and limits to set, is divided. It is, however, properly insistent that it has a role to play.

© 2015, Washington Post Writers Group