"Build up a navy" refers to the process of increasing the strength and size of a country's maritime forces. This can be achieved through a variety of means, such as constructing new naval vessels, recruiting and training more sailors, and investing in advanced weaponry and technology. Some synonyms for this phrase could include "expand the fleet," "strengthen maritime defense," "bolster naval capabilities," or "amplify seapower." Regardless of the wording used, the ultimate goal of building up a navy is to ensure that a country has the necessary resources to protect its interests and maintain its status as a global power.