Pakistan’s decision to go after the Afghan Taliban leadership reflects a quiet shift underway since last fall, said officials from both countries, who cited a November letter from President Obama to Pakistani President Asif Ali Zardari as a turning point.

The letter, which was hand-delivered by U.S. national security adviser James L. Jones, offered additional military and economic assistance and help easing tensions with India, a bitter enemy of Pakistan. With U.S. facilitation, the Indian and Pakistani foreign ministers have agreed to meet next week, the first high-level talks between the two countries since terrorist attacks in Mumbai in late 2008.

The letter also included an unusually blunt warning that Pakistan’s use of insurgent groups to pursue its policy goals would no longer be tolerated. The letter’s delivery followed the completion of a White House strategy review in which the administration concluded that stepped-up efforts in Afghanistan would not succeed without improved cooperation from Pakistan.

In explaining Pakistan’s shift, sources also cited regular visits to Pakistan by U.S. officials, a boost in intelligence-sharing and assurances by Washington that a military push in southern Afghanistan would not spill into Pakistan. The United States also promised Pakistani officials that it has no intention of abandoning the region once that offensive ends.