Has been ruled by the same family since 1948

The Kim dynasty of North Korea. The first Kim leader started to lead from 1948 and ever since then his descendants have been in power

the answer is north korea!!

Explanation:

John Quincy Adams signed the Florida purchase/Adams-Onís treaty in 1819, which officially ceded florida to the US and defined the boundary between the U.S. and New Spain. Spain gave Florida to the US because it had become a burden; they could not afford to send settlers or garrisons anymore.