Tuesday’s awful rush-hour bombings of trains in Bombay raise an important and ominous question: How far can India be pushed?

In December 2001 India and Pakistan almost went to war when a group of militants, based on Pakistani-controlled territory, attacked the Indian Parliament, killing nine people. India’s response was to mobilize forces along its border with Pakistan. Predictably and understandably, Pakistan followed suit. The U.S. State Department ordered all non-vital personnel out of both countries, and the world prepared for what could well have been the first war ever between two nuclear powers.

But due largely to extensive, active and exhaustive mediation by central figures from the West, tensions were ratcheted down, and in time forces were demobilized.

This time, it is not the West that needs to show leadership but the two countries themselves. They need to back up their words with actions. The leaders of India and Pakistan stated in April 2005 that “the peace process was now irreversible”; unless they both take action, this is now in question.

Three years ago, at first very quietly and with great sensitivity, India and Pakistan launched what was called the “composite dialogue.” The subjects ranged from economics to land to water to drugs to security. While many have suggested that these talks are going nowhere, they have led to some small but tangible progress.

Even “cricket diplomacy” has helped. Over the past two years, numerous matches in both countries have opened the eyes of the Indian and Pakistani populations to one another. They have found that those on the other side often think like them, look like them and even enjoy the same games.

More traditional benefits have also spun out of the dialogue. For the first time in more than 50 years buses are traveling between India and Pakistan, including across the Line of Control splitting the old state of Kashmir. Trains were recently started, and trucks, too. Visa restrictions have been relaxed, the militaries meet regularly, and, most notably, after the massive earthquake that struck Pakistan last October, India was one of the first countries to respond with offers of assistance (although the time taken to agree on the mode delayed action considerably).

This is all good. What hasn’t happened is arguably even more impressive. Despite an attack on a religious complex in Ayodhya last July, again by militants based in Pakistan, Indian Prime Minister Manmohan Singh announced that the dialogue would continue.

But – and here’s the crux of the matter – how long can India, Indians and the Singh government withstand the constant pressure from militant groups before they have to react? By any measure of international diplomacy, they’ve already been extraordinarily patient; compare their restraint with Israel’s response to the kidnapping of its soldier or to the U.S. and Japanese responses to North Korea’s missile tests.

Now is a moment when Pakistan really needs to respond. It wants to be taken seriously as an important player on the international scene. It has repeatedly asked the United States for a nuclear energy deal similar to the one we are working on with India. But until Pakistan – and this means not only President Pervez Musharraf but also the military, the people and the political parties, including the religious party, the MMA – gets serious about shutting down, arresting and otherwise dismantling the militant groups that operate from its territory, it cannot expect to be treated as a responsible player in the region. Pakistan is working on it, but it could do so much more.

A good – or at least stable – India-Pakistan relationship is one of the most important elements for long-term global stability. Given that both are nuclear powers, their region is one of the most dangerous in the world. And with attacks such as this, it is also one of the most volatile. India has taken great strides to tamp down this volatility. Pakistan needs to do more.

In return, India would need to step up in a real, substantive way on bilateral issues such as Kashmir. The third round of the high-level composite dialogue taking place next week, assuming it is still on, is the place to do it.

Xenia Dormandy is executive director for research with the Belfer Center for Science and International Affairs at Harvard’s Kennedy School of Government. She has served as director for South Asia at the National Security Council, a post she left last August. The views expressed here, first published in The Washington Post, are her own.