With China engulfed in chaos following the collapse of the Qing dynasty in 1911, Mongolian nationalists seized the opportunity to declared Outer Mongolia independent, with the support of Russia. In 1915 the Second Treaty of Kyakhta was signed, by which Russia acknowledged formal Chinese suzerainty over Mongolia, albeit with Russia maintaining significant influence, leaving the country in effect as a semi-autonomous condominium. Following the Russian Revolution in 1917 China invaded Mongolia in an attempt to re-assert full control, however they were ultimately repulsed by Mongol and Soviet Russian forces, with Mongolia declaring independence in 1921. China's refusal to recognise the country's independence meant that no formal border delimitation was conducted, though the remoteness and inhospitable, scarcely populated boundary terrain meant that this was in practice not a pressing issue. However, following Japan's invasion of Manchuria in 1931 and disputes over the Nomonhan region, Mongolia and the Japanese puppet-state of Manchukuo delimited a small section of their eastern frontier in 1935–39 in the vicinity of Buir Lake, though Japan was defeated in the Second World War leaving the status of this agreement in doubt.