For the loved ones of the 164 people who died in the 26/11 attacks, the arrest of Zabiuddin Ansari marks a small step forward in the slow march of justice. Mr. Ansari is alleged by India’s intelligence services to have been one of six men who guided the actions of a control team in Karachi. His arrest in Saudi Arabia will demonstrate to the masterminds of 26/11 that their crime is not forgotten, and that there will be no impunity. India’s intelligence services deserve credit for patiently pursuing Mr. Ansari — but so do its diplomats and national security strategists. India’s hunt for the perpetrators of 26/11 has met with more success than any past transnational terrorism investigation because of adroit use of the country’s geostrategic influence. New Delhi has succeeded in pushing the United States to take its counter-terrorism concerns seriously, hammering home the issue’s centrality to the relationship. Riyadh’s increasing cooperation with Indian counter-terrorism interests is also driven by its appreciation of the realities of a changing world. India, with China, will be among the kingdom’s principal energy markets in coming decades. More important, Saudi diplomats wish to wean India away from its long-standing relationship with their key regional adversary, Iran. There is one place, though, which remains shut off from the evolving international consensus on terrorism — and that is Pakistan, where 26/11’s key architects live.

There are three things India ought to do to deal with this situation. First, its high officials must resist the temptation to engage in the kinds of media-driven verbal India-Pakistan mud-wrestling that has periodically broken out over the 26/11 case. It has long been clear that powerful figures in Pakistan’s intelligence services had at least something to do with the attacks; it is unlikely that the country’s most powerful institution will collaborate in its own prosecution. It is not without reason, after all, that the men who harboured Osama bin Laden in Abbottabad are yet to be identified. Secondly, India must conduct the prosecution of Mr. Ansari as transparently as possible. Ill-advised Mumbai police investigators attempted to frame Indian nationals Sabahuddin Ahmad and Fahim Arshad Ansari for a role in the 26/11 attacks, disgracing themselves in the process. Finally, in years to come, with the influence of jihadist groups in Pakistan waxing, there will be new threats to India. New Delhi must anticipate and counter these threats using all tools at its disposal, from enhancing its counter-terrorism capabilities to doing what it can to strengthen the hands of democratic forces across the border.

More In: Editorial | Opinion