To make it a bit more interesting, we are gonna measure relative territory loss, expressed as a percentage, by comparing a country's current territory to its largest extent.
A 100% ratio indicates complete territory loss.
The British Empire was the largest empire ever built, with 35.5 million km^2 under its rule. Today, the UK's territory has shrunk to a mere 243.000 km^2. That means that the British have lost about 99.3% of their holdings.
The Mongols had 24 million km^2 of territory at their peak, but they currently retain only 1.57 million km^2, resulting in a territory loss of approximately 93.5%.
However, the country that has lost the most territory relative to its peak size is Vatican City. From a peak size of around 44.000 km^2, it has shrunk to become the smallest country in the world at only 0.49 km^2. This means that the Vatican has lost 99.999% of its territory.
