In the days of the Barbary pirates, the infant U.S. Navy swiftly avenged attacks on American lives and property. But after six Americans were among the more than 160 killed in the Mumbai terrorist attacks of November 2008, America's leaders wrung their hands and did nothing.

Two years later, despite clear evidence that the attack was carried out by the Pakistani terrorist group Lashkar-e-Taiba with the direct encouragement of Pakistan's intelligence agency, the United States has yet to press effectively for punishment of those responsible.

The evidence of the complicity of Islamabad's Inter-Services Intelligence, or ISI, has been clearly established by the FBI through its interrogation of a Pakistani American, Daood Gilani, who pleaded guilty in federal court to playing a key role in Mumbai. The case against Gilani, who used the name David Headley and spent part of his youth in Philadelphia, was spelled out in an exhaustive Washington Post-Pro Publica investigative report in November, which traced Headley's links to the ISI. The report identified his ISI controller as a "Major Iqbal" who operated through Sajid Mir of Lashkar-e-Taiba.

The FBI made Headley available to Indian intelligence agencies for interrogation in June. Confronted by Indian telephone intercepts, Headley reportedly identified Pakistan-based handlers from Lashkar and the ISI. Bruce Riedel, a former CIA officer who has had access to the Indian interrogation transcripts, has said there is "no question of an ISI role in Mumbai."

In his new book, Deadly Embrace: Pakistan, America and the Future of the Global Jihad, Riedel details what Headley told the Indian intelligence agencies. But much of what Headley said, Riedel explained to me in an interview, is not in the portions of Headley's guilty plea that the FBI cleared for publication.

"Headley's ISI contacts are not ruled out in the published transcript," Riedel told me, "but neither are they confirmed. Having read both his guilty plea and the records of the Indian interrogation of Headley, it is alarming that the guilty plea makes no mention of the ISI role in the Mumbai massacre. Why is this? The FBI would say because it's not sure of the ISI role. But did the FBI omit it from the guilty plea for what we might call 'diplomatic reasons'?"

Because the United States has not declared Pakistan a state sponsor of terrorism, there is little prospect of success for a pending lawsuit against the ISI by relatives of four of the American victims.

The United States should focus initially on obtaining an indictment of the suspected principal Pakistani culprit in the Mumbai attacks, Hafiz Muhammad Saeed. Following sanctions initiated by the Treasury Department in November, the U.S. government should also take further steps to target Lashkar-e-Taiba-linked American bank accounts.

Pakistan's casual attitude toward Lashkar was reflected in a recent comment at a Washington think tank by Ehsan ul-Haq, a former ISI director. Haq said that because Lashkar is "not attacking" Pakistan, "they're not a problem for us like the tribal terrorist groups on the border." Significantly, Gen. Ashfaq Kayani, now the chief of staff of Pakistan's army, headed the intelligence agency when Lashkar recruited Headley.

Confronting Pakistan on the matter of Lashkar-e-Taiba would require a credible threat to suspend all forms of military aid until Hafiz Muhammad Saeed is brought to justice and, more important, until all Lashkar training camps and storage depots in Pakistan and Pakistani-occupied Kashmir are destroyed.

What would the United States have to lose in making such a threat? Islamabad has already spurned all U.S. entreaties to stop military aid to Taliban militants who are killing our soldiers in Afghanistan. Pakistan is, for all practical purposes, an enemy of the United States in that war.

Equally important, the soft U.S. posture toward Lashkar-e-Taiba, whose avowed aim is to destabilize India, is incompatible with American efforts to strengthen ties with New Delhi. Even as Pakistan drifts toward anarchy, massive India is steadily building its economic and military potential, and it has emerged as a rising global power that will be of lasting importance to the United States.