At the end of the Mexican-American War, the United States annexed more than half of Mexico’s territory under the 1848 Treaty of Guadalupe Hidalgo. Under its terms, the U.S. promised to protect the property rights of Mexican citizens who had received land grants (ranchos) in the territory from Mexico or Spain. In 1851, Congress passed a law that required Mexican landowners in California to confirm their titles with the federal government. It was a lengthy and expensive process further complicated by different languages, laws, and customs. The cost of proving legitimate ownership was often the land itself.