In the early days of exploration a flag of ownership was placed upon new-found-lands to claim ownership, even though on the other side of the island, or bay there may have been another flag from a different country.
One problem with a flag – it doesn’t last very long.
But build a lighthouse and claim ownership and that light is visible to all peoples for years into the future. Build it high enough and it is visible for 360 degrees. Put some men on it and it becomes your property. Hmmm!
Right now in the news there are two island disputes in the South China Sea area that involve China and the Philippines – a stand-off over the Panatag Shoal (Huangyan Island; aka Scarborough Shoal) where China is contesting the Philippines’ internationally recognised exclusive economic zone, and China and Japan – an age-old dispute surrounding the group of islands called Senkaku by the Japanese and Diaoyu by the Chinese.