It is too often the rule that you fight most with those who are nearest to you. Since Partition in 1947, India and Pakistan have fought three fratricidal wars, nearly started a fourth and inched so close to a nuclear exchange that the borders separating them are today among the most dangerous - and militarised - in the world.
Nowhere is as tense as the Line of Control dividing the two country's claims in Kashmir, and at the weekend violence flared again. By the time the shooting had stopped, one Pakistani soldier was dead and another wounded. Indian officials denied reports their forces had crossed into Pakistan-controlled Kashmir to raid the Sawan Patra checkpost; Pakistani forces said an incursion started the violence.
Cross-border skirmishes and violations of the 2003 ceasefire in Kashmir are nothing new. Last year, there were more than 75 ceasefire violations along the Line of Control; most were just potshots traded across the border, but at least eight were killed. Over the past 25 years, as many as 100,000 people have died in the region - most of whom succumbed to the harsh winter weather that plagues the garrisons on both sides.
But an Indian-led incursion, if true, would be a significant provocation. The last major conflict in 1999 started after Pakistan invaded Indian-controlled territory in Kargil. Another conflict, which verged so closely to open war, would destabilise the entire subcontinent. With a war in Afghanistan raging and Pakistan struggling with its domestic demons, the region is in desperate need of detente.
Since the 2008 Mumbai attacks, relations between Indian and Pakistan have been improving, although at a glacial pace. Trade deals have been inked, and last month the two sides signed a plan to ease visa restrictions. There is a genuine commitment to better relations among political leaders in both New Delhi and Islamabad, although generals in both countries are sceptical given the long history of conflict and mutual suspicion.
The tragedy of this relationship, of cousins if not of brothers, is that all of the painstaking progress of diplomats and businessman can be overturned in an instant by violence. This was obvious after the Mumbai attacks and the assault on New Delhi's parliament in 2001, both of which were carried out by Pakistan-based militant groups. Kashmir's heavily militarised border, however, has always been a flashpoint.
A lasting solution in disputed Kashmir will take decades in the best-case scenario, and must take into account the will of the Kashmiris themselves. But political leadership is needed now. Border skirmishes - whoever is to blame, whatever the cause - risk the prosperity and security of two great nations that have far more in common than just this conflict.