World War I was a horrible bloodbath that the various European countries fell into almost before they knew what they were doing, due to a series of interlocking treaties. After a century that had mostly been peaceful, at least in terms of avoidance of large-scale wars, people were shocked when a major war erupted in 1914 and surprised that it lasted as long as it did. There had been so much technological and medical progress, so many leaps forward, between 1800 and 1900, that many Europeans confidently believed humankind was advancing to a more civilized state of being. Just as standards of living had been raised to remarkable levels during the century and once-common diseases had been wiped out with vaccines, so many people believed that large-scale wars were vestiges of a barbaric past that was mercifully ended.
World War I was a huge setback to people's hopes for a peaceful future. Its bloody futility and waste disillusioned many. Woodrow Wilson, however, did not want that war to have been fought in vain. He wanted some good to come out of it. He, like many others, hoped it would be the war to end all wars, a final shattering conflict ushering in a boundless era of peace. He hoped that the League of Nations, by getting countries together in partnerships and giving them a place to talk and practice diplomacy, could be a chief mechanism in averting future wars from breaking out. He believed it could be a civilized venue for countries to work out their differences without having to resort to violent conflict.
Ironically, despite Wilson's enormous faith in the idea of the League of Nations, the United States did not join it when it formed—and, of course, World War I was not the world's final conflict.