America's Founders were concerned about invasion. It was mentioned it four times in the Constitution, though the term was never explicitly defined.
Article I, Section 8, Paragraph 15: The Congress shall have the Power (to) provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions;
Article I, Section 9, Paragraph 2: The Privilege of the Writ of Habeas Corpus shall not be suspended, unless when in cases of Rebellion or Invasion the public Safety may require it.
Article I, Section 10, Paragraph 3: No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.
Article IV, Section 4, Paragraph 1: 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 domestics Violence.
So Biden is totally derelict in this regard. What can be done? Congress isn't going to impeach him. The courts can't force him to act.