The United States Constitution

Adopted September 17, 1787

Article II — The Executive Branch

Section 1 — The President

[…]

Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation:

“I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.”

[…]

Article IV — The States

[…]

Section 4 — Republican government

The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence.

Treaty of Guadalupe Hidalgo

signed February 2, 1848 (Treaty with Mexico, binding on the United States and Mexico)

Article V

[…]

The boundary line established by this article shall be religiously respected by each of the two republics, and no change shall ever be made therein, except by the express and free consent of both nations, lawfully given by the General Government of each, in conformity with its own constitution.

(See also the Gadsden Purchase.)

United States Code

Section 8, Chapter 12, Subchapter II, Part IV, Section 1227.a.1.B

Any alien who is present in the United States in violation of this chapter or any other law of the United States, or whose nonimmigrant visa (or other documentation authorizing admission into the United States as a nonimmigrant) has been revoked under section 1201 (i) of this title, is deportable.

(See more from the USC here.)
California Penal Code

Section 834b

(a) Every law enforcement agency in California shall fully cooperate with the United States Immigration and Naturalization Service regarding any person who is arrested if he or she is suspected of being present in the United States in violation of federal immigration laws.

(b) With respect to any such person who is arrested, and suspected of being present in the United States in violation of federal immigration laws, every law enforcement agency shall do the following:

(1) Attempt to verify the legal status of such person as a citizen of the United States, an alien lawfully admitted as a permanent resident, an alien lawfully admitted for a temporary period of time or as an alien who is present in the United States in violation of immigration laws. The verification process may include, but shall not be limited to, questioning the person regarding his or her date and place of birth, and entry into the United States, and demanding documentation to indicate his or her legal status.

(2) Notify the person of his or her apparent status as an alien who is present in the United States in violation of federal immigration laws and inform him or her that, apart from any criminal justice proceedings, he or she must either obtain legal status or leave the United States.

(3) Notify the Attorney General of California and the United States Immigration and Naturalization Service of the apparent illegal status and provide any additional information that may be requested by any other public entity.

(c) Any legislative, administrative, or other action by a city, county, or other legally authorized local governmental entity with jurisdictional boundaries, or by a law enforcement agency, to prevent or limit the cooperation required by subdivision (a) is expressly prohibited.


See also:

… and others.

So, why is there still a problem? Because the elite in this country have decided that the laws don’t matter when the laws conflict with what they want. See this “Timeline: How The Obama Administration Bypassed Congress To Dismantle Immigration Enforcement.”