Can any portion of an american state declare independence from its state and create a new state?
<p>I was reading <a href="https://en.wikipedia.org/wiki/Territorial_evolution_of_the_United_States" rel="noreferrer">Territorial evolution of the United States</a>. There it says</p>

<blockquote>
  <p>The northeastern region of New York, known as the New Hampshire
  Grants, declared independence as New Connecticut.</p>
  
  <p>New Connecticut was renamed Vermont.</p>
  
  <p>Vermont, which had been considered part of New York despite acting
  independently since 1777, was admitted as the fourteenth state.</p>
</blockquote>

<p>Can this legally happen today? Can any portion of an american state declare independence from its state and create a new state?</p>

<p>Not all by itself.  It requires the consent of the legislature of the existing state, as well as of the US Congress.  But if they all agree, then yes, it is possible.</p>

<p>US Constitution, Article IV, Section 3:</p>

<blockquote>
  <p>New states may be admitted by the Congress into this union; but no new states shall be formed or erected within the jurisdiction of any other state; nor any state be formed by the junction of two or more states, or parts of states, without the consent of the legislatures of the states concerned as well as of the Congress.</p>
</blockquote>

<p>You might like to read about how <a href="https://en.wikipedia.org/wiki/History_of_West_Virginia#Civil_War_and_split" rel="noreferrer">West Virginia was created out of Virginia in 1863</a>.  Of course the State of Virginia had already seceded, and the Confederate legislature obviously didn't consent, but the federal government recognized a separate "Restored" government of Virginia which did consent.</p>

united-states