In his request that Congress declare war on Germany, Woodrow Wilson essentially argued that the presence of non-democratic governments like that of Germany endangers democracy everywhere. The reason for this is that non-democratic countries are likely to engage in warfare in circumstances where democratic governments never would.
Wilson argues that democratic governments do not take actions that will endanger peaceful countries. He says that autocratic governments like that of Germany do these things because there is no check on the power of their leader. He goes on to say that there can never be true peace with autocratic nations. Such nations, he says, can never be trusted. Any attempt to make peace with autocratic governments will fail because those governments are motivated by selfish goals and only want their own power.
In essence, Wilson is saying that Germany is a threat to world peace and democracy because it is an autocratic country that will be willing to go to war against any other country whenever its rulers feel that it is in their own narrow, selfish interests.
Listen to the full text of the speech here: