It has been said the 20th century was like no other. Certainly, it can appear that way—war was more prevalent and far more destructive, technological advances were breathtaking and rapid, the general standard of living increased many times over in the developed world, and postmodernism rose to prominence in the wake of a shattered Modernist dream of progressive enlightenment and human actualization.
On closer inspection, however, these are pretty superficial changes. War only seemed more prevalent and destructive because the earth's population was exponentially larger; technological advances didn't develop overnight, and many of them were simply improvements on old ideas; the standard of living increased, but not for everyone and usually at a high cost; postmodernism was simply a new incarnation of anti-Christian philosophy.
There really is nothing new under the sun, and that is perhaps the most important lesson of the century that included everything from the War to End All Wars, to the New Deal, the New Society, and the Sexual Revolution. Humans will always be devoted to selfishness, pride, and havoc; the 20th century seemed to have more than its fair share of those things simply because there were so many more people than at any time previous.
Everything seemed to take on a more global scale in the last century. The First World War (1914-1918) was primarily a European affair (Japan and the U.S. were the only non-European combatant nations), but its effects were felt on every continent. This was even more true during World War II (1939-1945), when fighting actually took place on every continent (it was limited in North America and Australia, but it happened) and the war claimed 57 million lives worldwide, including both soldiers and civilians.
In between the wars came the Great Depression, an economic upheaval originating in the United States but affecting the whole world. This came on the heels of the Roaring Twenties, a morally chaotic period that gave birth to jazz, made marijuana popular, and laid the foundations of the so-called Sexual Revolution of the 1960s.
President Franklin D. Roosevelt implemented a series of government-subsidied programs to revitalize the economy collectively known as the New Deal, which many blame for introducing socialist doctrine into the U.S.A. Socialism was a great fear as Communism (seen as socialism's necessary next step) was not-too-slowly taking over Eastern Europe and moving into Asia.
It wasn't the New Deal that ended the economic crisis, though. World War II was the catalyst that upped production, necessitated a more muscular economy, and led the Western world out of its stupor and into technological advancement and monetary affluence. Incidentally, this very fact led to an intense debate between free market economists like F.A. Hayek (whose libertarian ideas are enjoying a resurgence currently) and the interventionist ideals of John Maynard Keynes and his ilk.
After the Second World War, everything seemed slightly off-kilter. Whole armies of civilian soldiers were released back to their countries of origin, and after the carnage they'd seen it was hardly surprising they spent the next decade forgetting about it, and amassing as much material wealth and comfort as was humanly possible.
Those areas most affected by the war had longer memories, of course. The work of rebuilding took a long time, and the scars of Nazi Holocaust and Japanese scorched-earth were visible for too long (some of them are still there). In response, people embraced a raw form of humanism, a hedonistic humanism built around postmodern amibiguity nevertheless shadowed by constant threat of nuclear war and annihilation.
Postmodernism is essentially the denial of modernist trust in human reason, and the assumption that nothing can be actually known coupled with a disbelief in absolutes of any kind. Technically, the term originally referred to art and architecture, but quickly spread to literature, then philosophy, and finally to the broader society. It's the attitude that motivated the '60s hippie movement, the deconstruction of morality, and the emphasis on pluralism as a positive force.
It's in this confusing atmosphere that we find ourselves now. Modernism had proclaimed the gospel of human progress: that, given enough education and liberty, mankind would eventually reach a highly evolved state of near- or total-perfection. The violence of the 20th century, including both World Wars, the Vietnam War, and countless genocides (Russia, China, Cambodia, Yugoslavia, etc.) largely put an end to such a vision.
Christians understand that God controls all things, but without such an orienting assumption it's easy to see how unbelievers would latch onto something like postmodernism, that essentially denies individual culpability and blames social constructs for the world's problems instead. Our job is to counter this essential despair and capitulation with the Good News of Christ's redemption, no matter how out of control and terrifying the world is or seems.