Here is an extract from USNORTHCOM’s published position (http://www.northcom.mil/Newsroom/Fact-Sheets/Article-View/Article/563993/the-posse-comitatus-act/). NORTHCOM is the Unified Command within DoD, that exercises command and control of US Military forces withing the Continental United States. They cite DODD 5525.5 as an implementing regulation within DoD, but the Departments of the Army and Navy also have their own implementing regulations.
As required by Title 10 USC, Section 375 the secretary of defense issued Department of Defense Directive 5525.5, which precludes members of the Army, Navy, Air Force, or Marine Corps from direct participation in a search, seizure, arrest, or other similar activity unless participation in such activity by such member is otherwise authorized by law.
The PCA generally prohibits U.S. military personnel from direct participation in law enforcement activities. Some of those law enforcement activities would include interdicting vehicles, vessels, and aircraft; conducting surveillance, searches, pursuit and seizures; or making arrests on behalf of civilian law enforcement authorities. Prohibiting direct military involvement in law enforcement is in keeping with long-standing U.S. law and policy limiting the military’s role in domestic affairs.
The United States Congress has enacted a number of exceptions to the PCA that allow the military, in certain situations, to assist civilian law enforcement agencies in enforcing the laws of the U.S. The most common example is counterdrug assistance (Title 10 USC, Sections 371-381). Other examples include:
The Insurrection Act (Title 10 USC, Sections 331-335). This act allows the president to use U.S. military personnel at the request of a state legislature or governor to suppress insurrections. It also allows the president to use federal troops to enforce federal laws when rebellion against the authority of the U.S. makes it impracticable to enforce the laws of the U.S.
Assistance in the case of crimes involving nuclear materials (Title 18 USC, Section 831). This statute permits DoD personnel to assist the Justice Department in enforcing prohibitions regarding nuclear materials, when the attorney general and the secretary of defense jointly determine that an “emergency situation” exists that poses a serious threat to U.S. interests and is beyond the capability of civilian law enforcement agencies.
Emergency situations involving chemical or biological weapons of mass destruction (Title 10 USC, Section 382). When the attorney general and the secretary of defense jointly determine that an “emergency situation” exists that poses a serious threat to U.S. interests and is beyond the capability of civilian law enforcement agencies. DoD personnel may assist the Justice Department in enforcing prohibitions regarding biological or chemical weapons of mass destruction.
Military support to civilian law enforcement is carried out in strict compliance with the Constitution and U.S. laws and under the direction of the president and secretary of defense.
It all BS. Soldiers throughout history have been paroling borders since there were borders and soldiers, i.e. for about 5,000 years or so. Apprehending a foreign national is not harming US citizens an is in the spirit of those regs.
We are in a lawless time so our side needs to use this to our advantage.