Forces from India and China clash on the border of the Arunachal Pradesh mountains

China and India share a disputed 3,440 km (2,100 miles) long de facto border – called the Line of Actual Control, or LAC – that is poorly demarcated. The presence of rivers, lakes and snow caps means the line can shift. The soldiers on both sides – representing two of the largest armies in the world – face each other in many places.