Is the United States at War?
<p>The US term GWOT (global war on terrorism) is frequently used, however, as I understand a nation is or is not at war (via declaration or some other formally documented mechanism) with another party.   In order for the US to be at war, what are the requirements for the other party?  In the distant past the party has been recognized nation states (Germany, Italy, Japan, etc.)   </p>

<h1>QUESTIONS</h1>

<ol>
<li>Is it possible for the US to be at war with a non-nation actor (ISIL, Al-Qaeda), in the same way that the US was at war with other recognized nations in WWII?   </li>
<li>If yes, has there been a declaration of war with XXXX (ISIL, Al-Qaeda, etc.)</li>
<li>Do the conventions of war apply to ideological combatants (i.e. not representing a recognized state)?</li>
</ol>

<p>Under <a href="https://www.law.cornell.edu/constitution/articlei#section8" rel="noreferrer">Article One, Section Eight</a> of the Constitution, </p>

<blockquote>
  <p>The Congress shall have power...</p>
  
  <p>To declare war</p>
</blockquote>

<p>but does not say what follows from "declaring war", nor does it say what form such a declaration shall take. <a href="https://fas.org/sgp/crs/natsec/RL31133.pdf" rel="noreferrer">This rather long document</a> analyzes the notion of "declaration of war". The primary significance of a "declaration of war" lies in international law, see the <a href="https://en.wikipedia.org/wiki/Hague_Conventions_of_1899_and_1907" rel="noreferrer">Hague Conventions of 1899 and 1907</a>. Articles 1 and 2 say:</p>

<blockquote>
  <p>The Contracting Powers recognize that hostilities between themselves
  must not commence without previous and explicit warning, in the form
  either of a reasoned declaration of war or of an ultimatum with
  conditional declaration of war</p>
</blockquote>

<p>and</p>

<blockquote>
  <p>The existence of a state of war must be notified to the neutral Powers
  without delay, and shall not take effect in regard to them until after
  the receipt of a notification, which may, however, be given by
  telegraph. Neutral Powers, nevertheless, cannot rely on the absence of
  notification if it is clearly established that they were in fact aware
  of the existence of a state of war</p>
</blockquote>

<p>So if the US were to declare war on Canada, we would need to officially notify the government of Canada of this fact, which we could do because there is a country Canada whose government we can inform. This is not possible with "terrorism", which is not a nation. There were official declarations for the War of 1812, against Mexico, Spain, and WW I, WW II (11 declarations). Since "the South" was not (from the perspective of "the North") a separate country, the United States did not declare war on itself to conduct the Civil War. In fact, since the congressional action authorizing military action against the Algerine Cruisers (the <a href="https://en.wikipedia.org/wiki/Second_Barbary_War" rel="noreferrer">2nd Barbary War</a>) in 1815, the actions authorized by Congress have not used the word "war" (except in mentioning a specific law, the War Powers Act), except in the aforementioned declared wars. Instead, the resolution described what could be done:</p>

<blockquote>
  <p>it shall be lawful fully to equip, officer, man and employ such of the
  armed vessels of the United States as may be judged requisite by the
  President of the United  States for protecting effectually the
  commerce and seamen thereof on the Atlantic Ocean, the Mediterranean
  and adjoining seas</p>
</blockquote>

<p>"Declared war" is a fairly limited genre of use of force, but "being at war" in the broadest sense, is common and can involve concepts and things (terrorism, poverty, drugs).</p>

<blockquote>
  <ol>
  <li>Is it possible for the US to be at war with a non-nation actor (ISIL, Al-Quaeda), in the same way that the US was at war with other
  recognized nations in WWII?</li>
  </ol>
</blockquote>

<p>Yes. Basically. As noted in the other answer it is usually not called a declaration of war if it is against a non-state actor.</p>

<blockquote>
  <ol start="2">
  <li>If yes, has there been a declaration of war with XXXX (ISIL, Al-Quaeda, etc.)</li>
  </ol>
</blockquote>

<p>Yes. It is called the <a href="https://en.wikipedia.org/wiki/Authorization_for_Use_of_Military_Force_Against_Terrorists" rel="noreferrer">Authorization for Use of Military Force</a> (AUMF) enacted September 18, 2001, one week after 9-11. This has been applied to ISIL because the executive branch officially determined as a matter of interpretation that ISIL is a successor organization to Al-Queada in Iraq which is one of the organizations covered by the AUMF. </p>

<blockquote>
  <p>Do the conventions of war apply to idealogical combatants (i.e. not
  representing a recognized state)?</p>
</blockquote>

<p>Yes, to the extent feasible. In the case of a de facto state, like ISIL, this is relatively easy to do. In the case of a more loosely organized terrorist organization it becomes increasingly difficult to apply the definition of core terms in international law like "enemy" and "combatant" and "non-combatant" to the facts on the ground.</p>

united-states laws-of-war