The decision by the Wilson Administration to enter World War I on behalf of the Allied Powers was motivated by a mixture of high-ideals and economic self-interest. There can be no doubt that President Wilson genuinely believed that American involvement in the war would help to make the world safe for democracy. At the same time, there were other, less altruistic concerns that he took into consideration before making such an important decision.
The increasing aggressiveness of German submarine activity in the Atlantic was having a damaging impact on American exports to Europe. The Germans attacked and sank a large number of US merchant ships, which inevitably caused consternation among American business interests, who became among the most vocal supporters of the United States' entry into the war. Though he didn't want war and had tried his best to keep the United States neutral in the conflict, Wilson could not ignore the growing clamor for action from the American business community.
Nor, for that matter, could he ignore public opinion. The rising tide of civilian casualties was arguably the biggest single factor behind growing support for US involvement in the war among the American people. The German sinking of the ocean liner Lusitania, which claimed the lives of nearly 1,200 people, had a particularly strong impact upon American public opinion, especially because many of those killed were American citizens.
Before long, support for US entry into the war became too strong for the Wilson Administration to resist, and so the United States formally entered the First World War on April 6, 1917.