World War I did not turn out the way that the countries thought it would because they were much too optimistic about the possibilities for offensive warfare given the technology that was available to the armies fighting the war.
At the beginning of WWI, most of the countries involved were very optimistic. As the war started in August, people predicted that it would be over by Christmas. They felt that they would be able to use cavalry offensively in a rapid war of movement. Both sides were confident that they would simply overrun the other.
The problem with this was that technology had progressed to the point where this kind of warfare was not going to be tenable. Things like rapid-fire artillery and machine guns gave tremendous advantages to the side that was fighting on the defensive. Once the French were able to dig in and create defensive lines, the German offensive ground to a halt and neither side was able to make any headway even as they launched many offensives.
The plans for WWI were made by people who did not realize how the technology of war had changed what was possible in terms of tactics and strategy. Therefore, their plans did not come to fruition.
By 1914 people and governments in countries of Europe felt that their country was the most powerful. On entering the war whether on the entente or alliances side they felt they would have won by the war ended. However, many countries did not realize that with the weapons they had it would have caused such massive destruction. One such country that did not anticipate this was Russia. By 1917, Russia had to withdraw from the war as a result of lack of resources. The Russian soldiers did not have enough weapons, food or uniform. All countries in and outside of Europe had challenges during the war and after 1918.
The social impacts or effects were millions of civilians and soldiers died, there was massive destruction of property, people were displaced, there was massive migration, technology increased and many persons became homeless. Economically, countries were in debt, a great depression emerged and there was massive inflation. Politically, a number of dependent countries gained independence and many countries changed their government systems.