"Despite formulating the concept and signing the Covenant, the United States never joined the League of Nations, and some relatively isolated sovereign states in Asia also did not join, including Saudi Arabia, Yemen, Mongolia, Nepal, and Bhutan." and "The League of Nations was established at the end of World War I as an international peacekeeping organization. Although US President Woodrow Wilson was an enthusiastic proponent of the League, the United States did not officially join the League of Nations due to opposition from isolationists in Congress."
To cause a source to be biased means that it is influenced by your opinion or another factor that is not factual. Bias can take away from the credibility because it is not completely true, as a source should be.
