Simplify the following text for a Grade 10 native English-speaking person.
Original: Although there were many European countries attending the congress , there were four major powers that controlled the decision making : United Kingdom , represented by her foreign secretary Viscount Castlereagh ; Austria , represented by the chief minister ( and chairman of the congress ) Klemens , FÃ 1\/4 rst von Metternich ; Russia , represented by Emperor Alexander I ; and Prussia , represented by King Frederick William III .
Simplified: These powers were the United Kingdom , Austria , Russia , and Prussia , represented by King Frederick William III .