The 1921 British Mount Everest reconnaissance expedition was the first to attempt to climb Mount Everest. It was followed by further British expeditions in 1922, 1924, and 1933. The climbers had to make the ascent from the north, since Nepal was closed. The situation became reversed after the Battle of Chamdo; expeditions launched after that had to use the southern approach through Nepal. The technical difficulties, especially in climbing the Second Step, were still unknown. There is ongoing discussion as to whether the Second Step was ever surmounted by George Mallory and Andrew Irvine in 1924. It was surmounted in 1960 as part of the first ascent of Mount Everest via the north route, when a shoulder stand was used to climb the last five metres.