The earliest source mentioning Stephen by name dates from 30 April 1455, when Pope Callixtus III put the King of Bosnia and his son under his protection. King Thomas's ambitions for Stephen grew as he strived to establish closer relations with the Western world. In 1456, he asked the Pope to procure a bride for his son, specifying that she should be a princess from a royal house. Negotiations soon commenced about Stephen's marriage to an illegitimate daughter of Francesco I Sforza, Duke of Milan, but Stephen's father had greater expectations.