Why are tensions along Afghan-Tajikistan border such a headache for China?

Monday’s fatal blast at a Chinese restaurant in Kabul has underscored the grave security threat China is facing, not just in Afghanistan but also in wider Central Asia. One potential area of vulnerability is Tajikistan, Central Asia’s poorest country, which has a weak military and a long border with Afghanistan. On the same day as the Kabul bombing, the Chinese embassy in Tajikistan issued a travel warning, telling its citizens and companies to boost security and evacuate from the border region...