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 country is Czechoslovakia,

It occurred in 1968. The Soviet Union led Warsaw Pact troops in an invasion of Czechoslovakia to crack down on reformist trends in Prague. The country was undergoing a series of reforms under the leadership of new communist party secretary general reformist Alexander Dubče.  The soviet union succeeded in the invasion, which they argued out as aimed at removing the remaining vestiges of capitalism.