I think it is time to clear the air and answer a fundamental question. Can President Trump, in view of the ongoing invasion at our southern border, declare an emergency and instruct our military to repel the invasion?
While the Posse Comitatus Act of 1878 forbids the use of U.S. troops inside the United States, the President can declare an emergency and use federal troops to enforce the law and protect our borders. See: Martin v. Mott, 25 U.S. 12 Wheat. 19 19 (1827)
“The power thus confided by Congress to the President, is, doubtless, of a very high and delicate nature. A free people are naturally jealous of the exercise of military power; and the power to call the militia into actual service is certainly felt to be one of no ordinary magnitude. But it is not a power which can be executed without a correspondent responsibility. It is, in its terms, a limited power, confined to cases of actual invasion, or of imminent danger of invasion. If it be a limited power, the question arises, by whom is the exigency to be judged of and decided? Is the President the sole and exclusive judge whether the exigency has arisen, or is it to be considered as an open question, upon which every officer to whom the orders of the [25 U.S. 19, 30] President are addressed, may decide for himself, and equally open to be contested by every militia-man who shall refuse to obey the orders of the President? We are all of opinion, that the authority to decide whether the exigency has arisen, belongs exclusively to the President, and that his decision is conclusive upon all other persons. We think that this construction necessarily results from the nature of the power itself, and from the manifest object contemplated by the act of Congress. The power itself is to be exercised upon sudden emergencies, upon great occasions of state, and under circumstances which may be vital to the existence of the Union.”
President Trump, by the terms of our Constitution, is empowered “To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions”.