In the writing of this thesis, I feel that I have worked with one of the most colorful and action-packed periods in the history of San Diego. The study sets aside a definite four-year period, 1846-1850, in the history of "California's first city." I have shown the important part played by San Diego as an American military base in the Mexican War. The pro-American feelings of the San Diegans are well-illustrated in this work. The military movements into San Diego occur in ordinary sequence. I believe this study will give the reader a better understanding of the military period in San Diego history, as well as that of California.