The speculation is that the terrorists were trying to destabilize the region to disrupt Pakistan’s help with al-Qaida (they might have to move their troops off the border with Afghanistan):

Relations between India and Pakistan were on a knife edge last night amid fears that Delhi’s response to the Mumbai attacks could undermine the Pakistani army’s campaign against Islamic militants on the frontier with Afghanistan.

Officials and analysts in the region believe that last week’s atrocities were designed to provoke a crisis, or even a war, between the nuclear-armed neighbours, diverting Islamabad’s attention from extremism in tribal areas bordering Afghanistan and thus relieving pressure on al-Qaeda, Taleban and other militants based there.
http://blog.beliefnet.com/cgi-bin/mt/mt.cgi?__mode=view&_type=entry&blog_id=72
One analyst even described the attacks as a “pre-emptive strike” against Barack Obama’s strategy to put Pakistan and Afghanistan at the centre of US foreign policy.
The United States and its allies now face a balancing act in supporting India’s efforts to investigate the Mumbai attacks, without jeopardizing Pakistan’s crucial support for the Nato campaign in Afghanistan.
India’s government, facing an election by May, is under enormous pressure to respond to the attacks, which it believes was carried out by the Pakistani militant group Lashkar-e-Taiba, possibly with the help of al Qaeda.

So, much for Obama bringing peace to the world. If you thought he could, you’re learning a very valuable lesson. We don’t control the war, the enemy does.