About once a month, the Central Intelligence Agency sends a fax to a general at Pakistan's intelligence service outlining broad areas where the U.S. intends to conduct strikes with drone aircraft, according to U.S. officials. The Pakistanis, who in public oppose the program, don't respond.

On this basis, plus the fact that Pakistan continues to clear airspace in the targeted areas, the U.S. government concludes it has tacit consent to conduct strikes within the borders of a sovereign nation, according to officials familiar...