Conversation so far:
When did Theodore Roosevelt become assistant secretary of the navy?
President William McKinley appointed Theodore Roosevelt as the Assistant Secretary of the Navy in 1897.
What did Theodore Roosevelt do as assistant secretary of the Navy?
Influenced by Alfred Thayer Mahan, Theodore Roosevelt called for a build-up in the country's naval strength, particularly the construction of battleships

Question: Why did he strengthen the navy?
Self-contained rewrite: Why did Theodore Roosevelt strengthen the Navy?