Prague Spring

"Prague Spring" was a real historical event:

...a period of political liberalization in Czechoslovakia during the era of its domination by the Soviet Union after World War II. It began on 5 January 1968, when reformist Slovak Alexander Dubček came to power, and continued until 21 August when the Soviet Union and members of its Warsaw Pact allies invaded the country to halt the reforms.

As metaphor, a Prague Spring is any temporary thaw in a regime of icy oppression.

Liberty broke through the ranks,
until the Soviets sent in tanks.