Argentina invaded the British island of Southern Thule in 1976.
The island was uninhabited and is so remote that the British weren’t aware of the invasion for over a month. When it was discovered that the Argentinians had set up a permanent military base and weather station there it was decided that diplomacy (secretly) was the smartest choice. It leaked to the press about a year later and Operation Journeyman was started to force a peaceful (albeit with the military) resolution.
It wasn’t until after the Falklands War ended that the British forced Argentina to leave (with shells fired at the island to force a surrender). But 6 months later an Argentine flag appeared on the island again.
How the British flag was replaced with the Argentinian one is unclear, although they did leave the British one neatly folded under a rock nearby. The British decided not to play the ‘replace the flag’ game and instead destroyed all the buildings (bar one small hut for emergencies) and left the British flag on the pole as they left.