Singapore was the effective capital of the British trading empire in the region. Governance, trade, and taxation flowed through Singapore for over a century and left it considerably wealthier and more developed than neighboring islands in the aftermath of WWII.
That positioned it well to continue acting as a business and political intermediary between western and eastern powers even after it became a self-governing state.
You can see similar levels of wealth concentration in other former imperial hubs like Hong Kong and Macau that were long used as intermediaries with China.
Latest Answers