1. Knowing the meaning of legitimate helps you determine that Frederick refused to acknowledge Marie Theresa as the legitimate empress of Austria because he believed that she did not have the right to rule according to accepted rules and standards.
2. A rivalry is a competition or a fight for superiority between two people or groups. In the context of "The War in Europe," the context clues that help determine the meaning of rivalry are the words "competition" and "fight for superiority." These words suggest that there is a struggle between two sides to gain an advantage over the other.