One long term effect of World War II was to reconfigure the world order. The core of the new world order was reconfigured around NATO in the west and the Soviet-led Warsaw pact countries in Eastern Europe. Britain, long in decline, finally lost its place for good as a world super power, a role taken over by the United States and the Soviet Union, who entered into a rivalry for influence and control over what we would today call Third World nations. Democracy and socialism definitively won over monarchy as governing institutions, and consumer societies in the west became firmly entrenched. Stable alliances between powerful nations such as the United States, France, Britain, and Germany insured peace in Europe over the long term. Israel was established as a state, and the age of imperialism definitively came to end, symbolized when Great Britain in 1947 handed back self governance to India, once the "jewel" in Britain's imperialist "crown."

Long term, too, the spread of nuclear capability forever changed the nature of warfare, with the "total war" of two superpower armies meeting on the battlefield of home countries replaced by a series of proxy wars in remote locales.

In the short term, the entire European continent, including Britain, had to deal with the devastation that war had wrought. Countries lay in ruins and had to be rebuilt. As the one remaining intact industrial power, the United States achieved an immense economic advantage over the rest of the world and broke away in standard of living to greatly surpass the rest of the world for a time.

Although international politics appear to be changing, we still live in the world order forged after World War II, one that has prevented the outbreak of a third world war or a devastating nuclear encounter.

The main short term effect of World War II was to destroy the power of many countries that had previously been relatively strong.  Before WWII, Japan was strong and had an empire.  Before WWII, Germany was a major power.  France was a major power with an overseas empire.  The UK was as well.  After WWII, all of those countries were significantly weakened.  Japan, Germany, and parts of France were devastated physically.  Japan lost its empire immediately.  France and the UK had their economies severely weakened by the war.  In the short term, these changes in the fortunes of nations were the most important effect of the war.

In the long term, there were at least two major effects.  One was that WWII led to the Cold War.  Because the US and the USSR were the only two major powers left in the world, and because their political and economic systems were antithetical to one another, they entered into a decades-long rivalry a few years after WWII ended.  The second major effect of WWII was to make the US the world’s dominant economic power.  All of the countries that had once been competitors to the US were devastated by the war.  The US homeland was not touched by the war and many things about the war (particularly the scientific and engineering advances that came with it) helped the US economy grow.

World War II, in the short term, reduced the number of effective empires in the world to two: the US and the Soviet Union.  The war bankrupted the British and occupied the French, with of course, Germany and Japan being completely defeated.  The American economy boomed both during and after the war, as the jobs and wealth it created took us out of the Great Depression and heavily industrialized the country.

In the long term, the economy remained healthy in the US and the middle class greatly expanded, but the Cold War struggle between the two remaining superpowers would dominate world affairs for half a century, divide countries in Europe and Asia, launch civil wars in dozens of places around the world and begin a very expensive nuclear and conventional arms race.  The atomic bomb invented during World War II could not be uninvented, and in the modern day nuclear proliferation is still a problematic leftover from World War II.

What were the short- and long-term effects of World War I?

Short-term effects of World War I (1914–1918) included the collapse of four empires, the enormous human and material losses caused by modern warfare, and the rise of non-democratic political systems.

World War I led to the collapse of Romanov Russia, Hapsburg Austria, Ottoman Turkey, and the German Empire. Defeated in war, the ruling dynasties of these four nations collapsed, and chaos ensued.

The carnage of World War I was much worse than that of any previous conflict in human history. Millions of soldiers were killed or maimed, and these men would have been among the leaders of their nations had they lived. The use of tanks and machine-guns had made war far more deadly. The severity of the losses made Anglo-French leaders overly timid during the interregnum between World War I and World War II. And the war cost was at least three hundred billion dollars in both direct and indirect expenses.

Political extremism appeared during and immediately after the war in Russia, Hungary, and Italy. Lenin established the Soviet Union, a Communist dictatorship. Bela Kun tried to do the same in Hungary. The Italian government was weak after the war, and this enabled Benito Mussolini to establish Fascism in Italy.

The mos -important long-term effect of the war was that it contributed to the rise of Adolph Hitler in Germany. Germany was forced to sign the Treaty of Versailles after World War I, and this discredited that country's postwar government—the Weimar Republic. Versailles, Germany's economic problems, and Hitler's shrewdness led to the creation of Nazi Germany, the Holocaust, and World War II.

What were the long-term effects of World War II?

One of the long-term effects of World War II, especially in the West, was the general acceptance of a much bigger role for government. The stubbornly high levels of unemployment that had persisted under FDR were only brought down after the United States entered the war in the wake of Pearl Harbor. Massive government involvement in wartime had virtually ended unemployment, so many came to believe that the same policy should apply in peacetime. If big government had helped win the war, why couldn't it help in peacetime?

There was a growing expectation among the general public that the government would use its considerable resources to help tackle some of the persistent social and economic problems that had plagued Western countries during the Depression years. In the United Kingdom, for example, Winston Churchill suffered a shock defeat in the 1945 General Election, comprehensively beaten by a Labour Party offering a radical socialist vision. The new government, led by Prime Minister Clement Attlee, embarked upon a massive program of government intervention, establishing a welfare state and a system of socialized medicine called the National Health Service.

For the better part of thirty years after the war, most people in Western society came to accept big government involvement in their lives as a matter of course. This would've been unthinkable in previous generations. But the Second World War had burnished the credentials of government as a positive force for good, one that could free people from the burden of want and allow them to live autonomous, dignified lives. It was only in the early 1970s, with the worldwide economic recession brought about by a sharp increase in the price of oil, that the prevailing consensus began to unravel.

The long-term effects of World War II were many, and as we discuss them, it is important to remember that the most important effect of the war was the over 50 million people, mostly civilians, who died during it. 

Long-term effects included the emergence of the United States and the Soviet Union as the world's two predominant superpowers. With Europe in ruins, these nations did much to shape the postwar order. Disputes between the United States and the communist Soviet Union over what this postwar order would look like fairly quickly escalated into what was known as the "Cold War," a struggle that spanned most of the rest of the twentieth century. This conflict saw the emergence of a Communist bloc in Eastern Europe and the division of Germany (and its old capital Berlin).

Another effect of World War II was the emergence of international institutions like the United Nations and the International Monetary Fund that were intended to maintain international law and stabilize the global economy. These institutions had been an aim of Woodrow Wilson and other leaders after World War I, but the new internationalist stance of the United States (another long-term effect of WWII) made it possible to establish them.

World War II led to the destabilization of the empires held by many European nations, including Great Britain. Many former colonies gained their independence, either in the face of European resistance or with European acquiescence. The independence of such nations as India, Indochina (Vietnam) and many former African colonies was a direct result of this trend. 

Within the United States, the war's end brought a significant demographic change known as the "baby boom" that changed the next half-century of American history. It ushered in unprecedented economic expansion and served as a starting point for major social change, including the civil rights movement for African-Americans.

These are only a few of many changes that emerged in the aftermath of World War II, many of which still have ramifications today.