The difference is that Outer Mongolia was occupied by Soviet troops in 1921, and formally declared independence in 1924, while Tibet did not follow Mongolia's path due to the decline of British power.
所不同的是，外蒙於一九二一年終被蘇聯軍隊佔領，而後於一九二四年正式宣告獨立；西藏則由於英國國力日衰而未覆蹈外蒙的途徑。