While I have viewed the matter through an originalist lens, others adopt different approaches. One prominent commentator, Marty Lederman, has an important explanation of the issues based on a conventional nonoriginalist perspective in these two posts. Here is an excerpt:
In the past two generations, there have been three principal schools of thought on the question of the President’s power to initiate the use of force . . . without congressional authorization:
a. The traditional view, perhaps best articulated in Chapter One of John Hart Ely’s War and Responsibility, is that except in a small category of cases where the President does not have time to wait for Congress before acting to interdict an attack on the United States, the President must always obtain ex ante congressional authorization, for any use of military force abroad. That view has numerous adherents, and a rich historical pedigree. But whatever its merits, it has not carried the day for many decades in terms of U.S. practice.
b. At the other extreme is the view articulated at pages 7-9 of the October 2003 OLC opinion on war in Iraq, signed by Jay Bybee (which was based upon memos written by his Deputy, John Yoo). The Bybee/Yoo position is that there are virtually no limits whatsoever: The President can take the Nation into full-fledged, extended war without congressional approval, as President Truman did in Korea, as long as he does so in order to advance the “national security interests of the United States.” With the possible exception of Korea itself, this theory has never reflected U.S. practice. . . .
c. Between these two categorical views is what I like to call the Clinton/Obama “third way”—a theory that has in effect governed, or at least described, U.S. practice for the past several decades. . . . The gist of this middle-ground view (this is my characterization of it) is that the President can act unilaterally if two conditions are met: (i) the use of force must serve significant national interests that have historically supported such unilateral actions—of which self-defense and protection of U.S. nationals have been the most commonly invoked; and (ii) the operation cannot be anticipated to be “sufficiently extensive in ‘nature, scope, and duration’ to constitute a ‘war’ requiring prior specific congressional approval under the Declaration of War Clause.”
As I have said before, I believe that the original meaning is similar to the traditional Ely view.
Lederman largely comes to the conclusion in this essay that a congressional declaration is needed under the practice, on the grounds that the practice has allowed unilateral presidential use of force only “in the service of significant national interests that have historically supported such unilateral actions—such as self-defense, protection of U.S. nationals, and/or support of U.N. peacekeeping or other Security Council-approved endeavors and mandates (e.g., Bosnia and Libya).”