The immigration system is like a jigsaw puzzle. If one or more pieces are out of whack, the puzzle makes no sense. To fix the system, Congress must make sure all of the pieces fit together, logically and snugly.

To do so, several realities must be faced squarely, including:

It is not law enforcement but the law itself that is broken. The nation has changed dramatically since the Immigration and Nationality Act of 1952, and that legislation has not held up well. It has been patched over so many times that it is hopelessly complex and incoherent. We need to start from scratch.

The nature of the border-security problem has evolved. The only tried-and-true method of reducing illegal immigration is a bad economy. Thus, with a dismal American economy and an improving Mexican one, the net immigration from Mexico is now zero: As many Mexicans are leaving the U.S. as are entering it.

The far greater border-security threat is paramilitaristic drug cartels that often are also involved in human smuggling, increasingly from Central American countries. The U.S. needs to coordinate closely with Mexico and focus its resources on defeating the cartels. The nation also needs to continue harnessing technology to identify risky foreign visitors and swiftly deport those who commit crimes or overstay their visas.