Article IV, Section 4 of the Constitution guarantees that the United States "shall protect each [State] against Invasion"–including invasions from immigrants illegally crossing our borders. 

As Alexander Hamilton wrote, "to admit foreigners indiscriminately...the moment they put foot in our country...would be nothing less, than to admit the Grecian Horse into the Citadel of our Liberty and Sovereignty." While I have compassion for those seeking to better their lives by coming to America, they must do so legally and with the full intention of assimilating into the American way of life.