Descriptions of California appearing in eastern newspapers had encouraged Americans to come and settle, and during 1846 immigrants had been arriving overland in greater numbers. The "Great Migration" of 1846 consisted of entire families, a completely different type of American immigrant than had arrived before. Stopping at Sutter's Fort upon completion of their journey, these newly arrived American immigrants were informed by Frémont and the Californian that they could shelter during the rainy season at a number of mostly unoccupied missions. Among those named was Santa Clara.

At Mission Santa Clara the immigrants would find a place ill-prepared to receive them. The years of being impacted by politics, stealing, and neglect since secularization, had impoverished what was once reputed to be the wealthiest mission in California. When visiting in 1848, Edwin Bryant described the picture of neglect he saw stating, "The rich lands surrounding the mission are entirely neglected... The picture of decay and ruin presented by a country so fertile and scenery so enchanting is a most melancholy spectacle to the passing traveler."

From mid-October through November 1846, an estimated 175 adults and children, including William Campbell and his family, arrived at Mission Santa Clara. Although upon their arrival they found a site in disrepair, due to the advent of the War the new arrivals decided to stay on at the compound. The immigrants sought shelter, living under what they would later describe as "deplorable conditions, sharing a large warehouse building with little light [the mission granary]. It was raining and the roof leaked. Food was in short supply." By the end of the year conflict arose. With few of the immigrants understanding Spanish or the customs and manners of the Californios, many offers of assistance were refused. Rumors transmitted as facts, prompted the organization of a militia at Santa Clara. One of the immigrants, Joseph Aram, established his headquarters at the mission with a force of thirty-one men assuming leadership when the mission militia elected officers. Ignoring the pleas of the Californios, Captain Aram and his men proceeded to cut down several of the willow trees (planted by Father Catalá) along the Alameda to use in barricading the mission. The lack of understanding between the two cultures culminated with the Battle of Santa Clara on January 2, 1847; the only campaign in the Northern District of California between the Californios and the United States forces during the Mexican-American war.

This "battle" which took place on the open plain about two miles from the mission, was a result of several rancheros rebelling against Americans taking their livestock and property. It was actually a 2 hour skirmish not a battle; no one was killed, and the only casualty was the American military forces' cannon, which continually bogged down in the knee-deep mud. A peaceful treaty was arranged on January 7, 1847. However, the American immigrants who viewed it from the tops of the mission buildings interpreted it as a tremendous defeat of the "enemy." Joseph Aram's militia company was disbanded on March 1st, and for the American immigrants, the winter spent at Mission Santa Clara was over. However, during 1847, problems would continue at Santa Clara due to a continuing influx of American immigrants; the non-Indian population of California almost doubled between 1845 and 1848. By Spring, immigrants were not only occupying the adobe buildings paying rent but many were simply "squatting" refusing to vacate the premises. In June Governor Mason ordered the unauthorized occupants to leave. However he proposed that the immigrants be allowed to stay until harvest time or longer if they paid rent, and Father Real, the last Franciscan priest of Mission Santa Clara, assented to that request.

William Campbell had enlisted as a private in "Captain" C. M. Weber's company of California riflemen, participating in the Battle of Santa Clara. In February he returned home to the Mission and among other enterprises took up the profession of surveyor. In October 1847, Father Real hired him to survey lots near the mission complex, on mission land, and draw up a town plat--this would become the Town of Santa Clara. It has been said at various times that this survey and its lots were later declared invalid, but the recordation of the first official survey in 1866 states differently: "this [1866] map... correctly represents the blocks, streets, and squares of the said town as surveyed in the year 1847; and...the land embraced with in the said survey of 1847 has been occupied and used for town purposes ever since."

Hostilities between the United States and Mexico ceased in early 1848, and the signing of the Treaty of Guadalupe Hidalgo on February 2nd, ceded Texas, New Mexico and California to the United States. No longer a Mexican Province, California was now an American possession, and Mission Santa Clara, an embryonic American town.

1. Mission Santa Clara, church and compound; located at the end of Palm Drive, Santa Clara University.
2. Mission Corral Plaque (where both side's horses were corralled after the Battle of Santa Clara); located in front of the Mission Library.