- California's strategic location. California is located on the Pacific Ocean, which gives it access to important trade routes and military bases. This makes California a key location for defending the United States against attack from overseas.
- California's resources. California has a wealth of natural resources, including oil, timber, and minerals. These resources were essential for the United States during the 19th century, when the country was rapidly expanding its economy and military.
In addition, some Americans believed that annexing California would help to prevent the United States from becoming a slave-holding nation. California was admitted to the Union as a free state, which would have reduced the number of slave states in the United States. This would have made it more difficult for the South to secede from the Union and start a civil war.
Overall, there were a variety of reasons why some Americans thought that making California part of the US would strengthen national security. These reasons included California's strategic location, its resources, and its potential to prevent the United States from becoming a slave-holding nation.