What basis did Executive Order 9066 have in preexisting legislation?
<p>From various Supreme Court rulings, we can see that "The President's power, if any, to issue [an Executive] order must stem either from an act of Congress or from the Constitution itself." - Mr. Justice Black, majority opinion for Youngstown Sheet &amp; Tube Co. v. Sawyer, 343 U.S. 585 (1952).</p>

<p>However, various Executive Orders didn't seem to have their basis in preexisting legislation or in the constitution: Namely, Executive Order 9066. Despite that, this Executive Order was still deemed constitutional by the Supreme Court.</p>

<p>What basis, if any, did that order have in legislation or the US Constitution?</p>

<p>The core order and authorization as stated in the order is:</p>

<blockquote>
  <p><em>by virtue of the authority vested in me as President of the United States, and Commander in Chief of the Army and Navy</em>, I hereby
  authorize and direct the Secretary of War, and the Military Commanders
  whom he may from time to time designate, whenever he or any designated
  Commander deems such action necessary or desirable, to prescribe
  military areas in such places and of such extent as he or the
  appropriate Military Commander may determine, from which any or all
  persons may be excluded, and with respect to which, the right of any
  person to enter, remain in, or leave shall be subject to whatever
  restrictions the Secretary of War or the appropriate Military
  Commander may impose in his discretion.</p>
</blockquote>

<p>A related order, <a href="https://fas.org/irp/offdocs/eo/eo-8972.htm" rel="nofollow noreferrer">8972</a>, uses the same phrase for a related purpose:</p>

<blockquote>
  <p><em>by virtue of the authority vested in me as President of the United States, and Commander-in-Chief of the Army and Navy of the United
  States</em>, I hereby authorize and direct the Secretary of War</p>
</blockquote>

<p>Truman's <a href="https://biotech.law.lsu.edu/cases/nat-sec/eo-10340.htm" rel="nofollow noreferrer">Executive Order 10340</a> similarly ordered</p>

<blockquote>
  <p>by virtue of the authority vested in me by the Constitution and laws
  of the United States, and as President of the United States and
  Commander in Chief of the armed forces of the United States...</p>
</blockquote>

<p>to seize steel plants, in light of an strike. In that particular case, SCOTUS ruleds in <a href="https://supreme.justia.com/cases/federal/us/343/579/" rel="nofollow noreferrer">Youngstown Sheet &amp; Tube Co. v. Sawyer</a>, 343 U.S. 579 that he did not actually have that authority. There is a difference between claiming an authority and having that authority validated by the courts. In <a href="https://supreme.justia.com/cases/federal/us/320/81/" rel="nofollow noreferrer">Hirabayashi v. United States</a>, 320 U.S. 81, one of the holdings was that</p>

<blockquote>
  <p>By the Act of March 21, 1942, Congress ratified and confirmed
  Executive Order No. 9066, and thereby authorized and implemented such
  curfew orders as the military commander should promulgate pursuant to
  that Executive Order.</p>
  
  <p>It was within the constitutional authority of Congress and the
  Executive, acting together, to prescribe this curfew order as an
  emergency war measure</p>
</blockquote>

<p>(Note that the order preceded the congressional authorization referred to: the curfew order is not part of EO 9066, but is a consequence of it).</p>

<p>In general, you cannot know if an order is <em>really</em> authorized by Congress or the Constitution until the Supreme Court rules on the question. This has happened only a handful of times, notably 5 times in Roosevelt's case.</p>

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