Had Henry Clay become President in 1845, he would undoubtedly have managed to placate Mexico, and with no Mexican War there would have been no Civil War, at least not in 1861.
But would Clay have acquired California?
California! The very name connoted mystery and romance. It had been given to a mythical kingdom "near the terrestrial paradise," in a Spanish novel of chivalry written in the lifetime of Columbus. President Polk did not read novels, but he wanted California much as Don Quixote wooed Dulcinea, without ever having seen her, and knowing very little about her. The future Golden State, with forests of giant pines and sequoias, broad valleys suited for wheat and narrow vales where the vine flourishes, extensive grazing grounds, mountains abounding in superb scenery and mineral wealth, was then a Mexican province, ripe for the plucking.
My vague impression is that California loomed largest in the national imagination in the years around 1964. The introduction of commercial domestic jet travel in 1959 integrated California, which had previously been thought of by East Coast elites as attractive but remote (somewhat the way Australia and New Zealand are thought of today), into the life of the leadership class.
Today, however, the word "California" is usually not used with an exclamation point.