Which nation had a massive warsaw pact army (led mostly by the soviets) of nearly 600,000 thousand soldiers sent into it to remove it's leader and stop reforms?

The answer is Czechoslovakia

Under the Danube Operation, an invasion of the then nation-state of Czechoslovakia occurred on the 20th of August 1968.

5 Warsaw Pact nations took part in the invasion including, Soviet Union, Poland, East Germany, Bulgaria and Hungary.

The forces and offense was led by Soviet Union.

The aim of the invasion was to Alexander Dubček's Prague Spring liberalisation reforms and provide more support and power to local communist political parties.

What research did you learn from?

Its is d. because i did it