Customs and Border Protection (CBP)
"[Custom and Border Protection's] priority mission of keeping terrorists and their weapons out of the U.S. It also has a responsibility for securing and facilitating trade and travel while enforcing hundreds of U.S. regulations, including immigration and drug laws."
"Border security is one piece of the very large controlling-immigration puzzle. But policing borders, including the use of physical barriers where necessary, has been integral to the preservation of national sovereignty for centuries. In our country, some two-thirds of the illegal population has snuck across the border with Mexico; the rest entered legally — as tourists, students, etc. — and never left."