Which country changed sides about a year after world war i began and joined the allies?

Me! these questions are based on american people in the south essential questions: question 1: for what reasons will one group of people exploit another?focus questions: question 1: what influenced the development of the south more: geography, economy, or slavery?question 2: what were the economic, political and social arguments for and againsts slavery in the first half of the 19th century.