Everything was quiet until the end of April, when we heard that the Tibetans were occupying a wall in some strength near the Karo la, forty-two miles from Gyantse, on the road to Lhasa. Colonel Brander, of the 32nd Pioneers, who was left in command at Gyantse, sent a small party of mounted infantry and pioneers to reconnoitre the position. They discovered 2,000 of the enemy behind a strong loopholed wall stretching across the valley, a distance of nearly 600 yards. As the party explored the ravine they had a narrow escape from a booby-trap, a formidable device of Tibetan warfare, which was only employed against our troops on this occasion. An artificial avalanche of rocks and stones is so cunningly contrived that the removal of one stone sends the whole engine of destruction thundering down the hillside. Luckily, the Tibetans did not wait for our main body, but loosed the machine on an advance guard of mounted infantry, who were in extended order and able to take shelter behind rocks.
On the return of the reconnaissance Colonel Brander decided to attack, as he considered the gathering threatened the safety of the mission. The Karo Pass is an important strategical position, lying as it does at the junction of the two roads to India, one of which leads to Kangma, the other to Gyantse. A strong force holding the pass might at any moment pour troops down the valley to Kangma, cut us off in the rear, and destroy our line of communications. When Colonel Brander led his small force to take the pass, it was not with the object of clearing the road to Lhasa. The measure was purely defensive: the action was undertaken to keep the road open for convoys and reinforcements, and to protect isolated posts on the line. The force with the mission was still an 'escort,' and so far its operations had been confined to dispersing the armed levies that blocked the road.
On May 3 Colonel Brander left Gyantse with his column of 400 rifles, comprising three companies of the 32nd Pioneers, under Captains Bethune and Cullen and Lieutenant Hodgson; one company of the 8th, under Major Row and Lieutenant Coleridge, with two 7-pounder guns; the Maxim detachment of the Norfolks, under Lieutenant Hadow; and forty-five of the 1st Mounted Infantry, under Captain Ottley. On the first day the column marched eighteen miles, and halted at Gobshi. On the second day they reached Ralung, eleven miles further, and on the third marched up the pass and encamped on an open spot about two miles from where the Tibetans had built their wall. A reconnaissance that afternoon estimated the enemy at 2,000, and they were holding the strongest position on the road to Lhasa. They had built a wall the whole length of a narrow spur and up the hill on the other side of the stream, and in addition held detached sangars high up the steep hills, and well thrown forward. Their flanks rested on very high and nearly precipitous rocks. It was only possible to climb the ridge on our right from a mile behind, and on the left from nearly three-quarters of a mile. Colonel Brander at first considered the practicability of delaying the attack on the main wall until the Gurkhas had completed their flanking movements, cleared the Tibetans out of the sangars that enfiladed our advance in the valley, and reached a position on the hills beyond the wall, whence they could fire into the enemy's rear. But the cliffs were so sheer that the ascent was deemed impracticable, and the next morning it was decided to make a frontal attack without waiting for the Gurkhas to turn the flank. No one for a moment thought it could be done.