A PROMISE to California, Also to the great Pastoral Plains, and for Oregon: Sojourning east a while longer, soon I travel toward you, to remain, to teach robust American love;
For I know very well that I and robust love belong among you, inland, and along the Western Sea; For These States tend inland, and toward the Western Sea—and I will also.
My first interpretation of "A Promise To California" is that Walt Whitman believes that California is the promise land in a way. He describes the beauty, importance, and location of the state. He also said he will be trying to get there and rest once he arrives.
My second interpretation of "A Promise To California" is that Walt Whitman wants to spread his love. I say this because this was written around the Civil War so tensions were high between states. So maybe he wanted to spread his love throughout the land crossing all the states to get to California.