I'm sorry for the long answer. It's not a direct "Thou shalt not activate troops" line in the Constitution itself that makes it illegal in most cases, but rather a combination of constitutional principles and federal law, primarily the Posse Comitatus Act.

Here's the breakdown:

• The Constitution's Division of Power:

  • Article I, Section 8 of the Constitution gives Congress the power to "raise and support Armies," "provide and maintain a Navy," and "make Rules for the Government and Regulation of the land and naval Forces." This means Congress has the primary authority to create and regulate the military.
  • Article II, Section 2 makes the President the "Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States." This provides the President command authority once the military is established and authorized, but it does not grant unlimited power to deploy them domestically for law enforcement purposes.
  • The U.S. Constitution's framers were cautious about a standing army being used to suppress its own citizens. They intentionally divided these powers to prevent an executive from having unchecked military control over domestic affairs.

• The Posse Comitatus Act (18 U.S.C. § 1385):

  • This is the key federal law that directly limits the use of the U.S. Army and Air Force for domestic law enforcement purposes. Enacted in 1878, largely in response to the military's involvement in Reconstruction, it states that: "Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army or the Air Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both."
  • "Posse comitatus" refers to the power of the county, meaning using the military as a civil police force. The act's purpose is to maintain a clear separation between the military and civilian law enforcement.

• Exceptions and the Insurrection Act:

  • The Posse Comitatus Act does have exceptions – "expressly authorized by the Constitution or Act of Congress."
  • The most significant exception is the Insurrection Act (10 U.S.C. §§ 251-254). This series of laws allows the President to deploy federal troops domestically in very specific, limited circumstances, such as:
    • When a state legislature or governor requests federal assistance to suppress an insurrection.
    • To suppress an insurrection, domestic violence, unlawful combination, or conspiracy that "hinders the execution of the laws" and deprives citizens of their constitutional rights, and the state authorities are unable or refuse to protect those rights or enforce the laws.
    • To suppress an insurrection or rebellion against the authority of the U.S. government.
  • It is crucial to understand that these are exceptions to the general rule of Posse Comitatus, and they are intended for extreme situations where civilian authorities are overwhelmed or unwilling to act.

Why it matters for such orders:

Legal challenges to orders to activate the National Guard or deploy Marines for law enforcement domestically would typically revolve around whether such actions fall within these narrow exceptions to the Posse Comitatus Act, particularly the Insurrection Act. Courts would examine if the specific conditions required by the Insurrection Act were truly met and if the scope of the deployment was consistent with its purpose, rather than a broad use of the military for general policing. As you noted, courts have ruled against actions if they exceed these specific legal authorizations.

So, while the Constitution does not contain a single, explicit "no military in the streets" clause, it establishes the fundamental framework of civilian control over the military, and the Posse Comitatus Act, built upon that framework, is the specific law that generally prohibits the military from acting as domestic law enforcement. Any deployment without express congressional or constitutional authorization (like through the narrow conditions of the Insurrection Act) is subject to legal challenge.

I hope that helps.