Some of the most sweeping and spectacular rhetoric of the left occurred in 18th century France, where the very concept of the left originated in the fact that people with certain views sat on the left side of the National Assembly.
The French Revolution was their chance to show what they could do when they got the power they sought. In contrast to what they promised -- "liberty, equality, fraternity" -- what they actually produced were food shortages, mob violence and dictatorial powers that included arbitrary executions, extending even to their own leaders, such as Robespierre, who died under the guillotine.
In the 20th century, the most sweeping vision of the left -- Communism -- spread over vast regions of the world and encompassed well over a billion human beings. Of these, millions died of starvation in the Soviet Union under Stalin and tens of millions in China under Mao.
Milder versions of socialism, with central planning of national economies, took root in India and in various European democracies.
If the preconceptions of the left were correct, central planning by educated elites with vast amounts of statistical data at their fingertips, expertise readily available, and backed by the power of government, should have been more successful than market economies where millions of individuals pursued their own individual interests willy-nilly.
But, by the end of the 20th century, even socialist and communist governments began abandoning central planning and allowing more market competition. Yet this quiet capitulation to inescapable realities did not end the noisy claims of the left.
In the United States, those claims and policies reached new heights, epitomized by government takeovers of whole sectors of the economy and unprecedented intrusions into the lives of Americans, of which ObamaCare has been only the most obvious example.