Once upon a time, the Left believed that history was on the march toward socialism. This belief has been shaken by various events, from the Moscow show trials, Hungary 1956, Czechoslovakia 1968, to the end of the USSR and restoration of capitalism in China.
Once upon a time, the Right believed, like Francis Fukuyama, that history had ended, with the culmination of history in capitalism + liberal democracy. This belief has been shaken by various events: Islamic resistance to the West's politico-military campaign to create a New Middle East, a resurgence of the Left in Latin America, the West's self-destruction of its own liberal and democratic facade through the "War on Terror," and finally the global financial crisis starting in the subprime states of America.
Now both sides have to approach history as an open-ended process of struggle.