American presidents such as George W. Bush, along with his foreign policy that had questionable results, and Woodrow Wilson, who brought the U.S. into the First World War before using it as an excuse to make detrimental changes to the country, are generally considered to be some of the worst cases based on the effects of their actions. Other American presidents also have negative reputations due to their actions while in office.
James Buchanan could also be considered one of the worst presidents, mainly as a result of his lack of action in dealing with slavery and the associated problems that gripped the country at the time. This is often referred to as a failure of omission.
Franklin Pierce has also been listed as one of the worst presidents. This is mostly due to the fact that he was often perceived to be in favor of the expansion of slavery in the south, and some unpopular deals he tried to broker with Spain and Great Britain during his presidency, such as attempting to persuade Spain to sell Cuba, for example.
Woodrow Wilson's actions, which led him to become one of the most unpopular presidents, can be referred to as a failure of commission. The problems the country faced as a result of Wilson's decisions were a direct result of his actions, but he failed to rectify them.