India lost Aksai Chin to Chinese in early 1950s when PLA built roads through it. Aksai Chin had been part of Ladakh which had gone to India in 1948 via Instrument of Accession. Of course the Indians never got anywhere the
15:1 advantage or Banglas to join them as auxiliaries so they did nothing. India just accepted Aksai as "lost" and went quite over it. This is something Indians don't like talking about.
https://en.wikipedia.org/wiki/Aksai_Chin
Britain, Portugal lost whole host of colonies. France lost Algeria after bitter decade long war. Russia lost all satellite states like Ukraine, Latvia, Kazakstan etc in early 1990s. Sudan lost South Sudan just recently. Indonesia lost Timor etc etc etc.
Of course to Indian fanboy history only Pakistan lost snce 1947.