California and Oregon never seceded from the Union during the Civil War, and presumably contributed in some capacity to put down the rebellion between 1861 and 1865.
But being separated by the eastern states by the Rockies, before there was a Trans-Continental Railroad, and before the Panama Canal made for faster shipping between the American coasts, it must have been very difficult for these two states (and then Nevada in 1864) to contribute substantially.
What exactly did these Western states contribute to the war effort for the Union? Troops? Supplies? Did California or Oregon men play key roles in battles or campaigns? Would love to know!
Many immigrants went to the West to avoid the draft: in all practicality, the federal government could not call on westerners to serve. That was part of the motivation of Samuel Clemens when he went with his brother, Orion, to the Nevada territory in 1861: Clemens (soon to take the name Mark Twain) did not want to be drafted by Union forces to serve as a riverboat pilot. Westerns followed the conflict, and the population was dominated by pro-Union sentiment. But the distance meant that they could do little but wait for news of battles. Some went back to "the states" to serve, but the logistics meant that most did not. There was a famous fundraising effort involving the Gridley sack of flower that was repeatedly auctioned for the precursor of the Red Cross: it raised hundreds of thousands of dollars, and it reflected the widespread desire to support the effort.