What is the fine-grained topic of the following text: When U.S. filibuster William Walker overthrew the government of Nicaragua and attempted to conquer the other nations in Central America, including Costa Rica, in order to form a private slave-holding empire, Costa Rican president Juan Rafael Mora Porras called upon the general population to take up arms and march north to Nicaragua to fight against the foreign invader. Only return the topic.
William Walker's invasion of Central America and Costa Rica's response