Why did Polk urge war?

Answer 1

As he told his Cabinet and wrote in his diary, Polk wanted above all else to acquire California for the United States, mainly to prevent the militarily strategic Bay of San Francisco falling into British hands. He wanted to avoid war if this was possible. In late 1845 he sent diplomats to Mexico City to purchase California in exchange for eliminating the several debts that American merchants claimed they were owed for ship cargoes seized by Mexico during the previous 10 years. The American diplomatic effort failed because the idea of surrendering more land was extremely unpopular. Polk now sent troops into the land that the United States regarded was part of western Texas which they had just acquired. Mexico was not prepared to cede this land to the United States and defended it militarily. The killing of American soldiers on what the United States regarded as their territory allowed Polk to announce to Congress in 1846 that a state of war already existed and more troops would be needed to be sent to Texas and into Mexico. Polk sent messages to his navy in the Pacific to immediately seize Californian ports and hold onto them so that whenever a peace treaty was signed with Mexico, the United States would already be in possession of California. By the end of the war the United States was in possession of not only California, but also Baja Californa, and most of Mexico including Mexico City. From a position of strength they could now negotiate what they wanted to keep.