One important lesson our country learned on Sept. 11, 2001, is that state and local police can make the difference between an unsuccessful terrorist plot and an attack that kills nearly 3,000 people. But some in Washington, D.C., still have not absorbed this lesson.

The immigration bill (S. 2611) approved by the Senate last month strips local police officers of arrest authority that could have been used to stop the 9/11 attacks.

In the aftermath of 9/11, we learned that five of the 19 hijackers had violated federal immigration laws while they were in the United States. In other words, they were illegal aliens. Amazingly, in the months before the attack, four of those five terrorists were stopped by local police for speeding. All four could have been arrested—if the police officers had realized that they were illegal aliens...