James k polk hoped to gain access to the pacific ocean through

Purchase of California from Mexico--Polk first offered Mexico money to purchase California and that land would give the US access to the Pacific. Mexico rejected the offer. 

The rejection of the purchase offer, led to Polk taking more extreme measures by moving troops into Texas on the Rio Grande. This land was disputed and viewed by Mexico as an attack. The Mexican-American War would start with this aggression and end with the US gaining the land they wanted and more from Mexico.