Attack helicopters supporting International Security Assistance Force (ISAF) troops on the Afghan-Pakistani border reportedly fired upon a Pakistani Frontier Corps position Sept. 30, killing three paramilitary Frontier Corps troops and wounding three others. According to Pakistani media reports, there have been two incidents of ISAF attack helicopters engaging targets in Pakistan. One took place before dawn and one at 9:30 a.m. local time, both northwest of Parachinar, the main town in the Kurram agency of Pakistan's Federally Administered Tribal Areas, according to preliminary reports. The ISAF forces were operating in the Dand Patan district of Afghanistan's Paktia province. ISAF has claimed that its troops were responding to mortar fire and remained on the Afghan side of the border, and it believes that at least one of the two places engaged by close air support could have been on the Afghan side of the border. The Pakistani government quickly issued strong condemnation of the incident. (click here to enlarge image) There is no shortage of potential scenarios for what actually happened on the ground. ISAF troops are regularly engaged from the Pakistani side of the border, and cross-border exchanges of fire and fighting on the border are common. ISAF may have even been fired upon from the Frontier Corps position. Or it may have been an error on the ISAF's part and the Frontier Corps position was accidentally or inappropriately engaged. Pakistan has suggested that the Frontier Corps position was deliberately engaged. But the facts in this case are really beside the point. According to a well-placed STRATFOR source in Pakistan, the Pakistani army's General Headquarters considers this the fourth incident in less than a week — and the most offensive because the Pakistanis believe their troops were directly targeted. Just two days earlier, Pakistan warned that it would stop protecting ISAF supply lines to Afghanistan if foreign aircraft continued engaging targets across the border. Following through on that threat, the Pakistanis closed the border crossing over the Khyber Pass at Torkham in response to the Sept. 30 incident. It is not yet clear how long the border will remain closed in protest. Short disruptions are completely manageable logistically in Afghanistan and have been accommodated in the past. But the government in Islamabad has been feeling increased pressure as U.S. unmanned aerial vehicle strikes on militant positions in Pakistan's tribal areas have increased, and widespread domestic dissatisfaction with the response to the humanitarian disaster caused by flooding earlier this year has only further strained the government. Domestically, Islamabad has little room to compromise or back down on this. Moving forward, the key issue is not the facts of this particular incident, but the Pakistani government's response — essentially whether this is largely for show, and what Islamabad demands of the United States operationally. At this stage it is unclear how long this situation will persist but it is very likely that the move to block the supply route was designed to force the United States to back off from the latest wave of cross-border operations.