Answer
Stalinism, the answer to the question, was the ideology challenged by Dubček's reforms. This ideology, characterized by strict centralized control, suppression of dissent, and a strong emphasis on state security, was dominant in Czechoslovakia during the 1950s and early 1960s. Dubček's reforms aimed to liberalize the country's political system, decentralize decision-making, and introduce elements of freedom of expression and assembly, thereby challenging the fundamental principles of Stalinism.