Power and ambition are closely connected. People who have power but do not have any ambitions might lose their position to someone who is simply more ambitious.
An ambitious person is someone who has a strong mindset and who will execute the necessary steps in order to achieve a desired goal. An ambitious person will keep making progress in life, work well with people and will naturally hold a position of leadership. People who have both power and ambition can maintain their high position and persevere against any schemes to take away their position.
In Othello, Iago is a very ambitious man. By manipulating others, he gains resources, such as money (from Roderigo). His selfish ambitions lead to a disastrous ending for the main characters (Othello and Desdemona) as well as himself. Unlike Iago, Othello actually has power. However, since he does have a high self-esteem, this makes him an easy target for Iago’s manipulation.