Their leader assassinated and their homes under attack, the members of the Church of Jesus Christ of Latter-day Saints (also known as Mormons) of Nauvoo, Illinois, begin a long westward migration that eventually brings them to the valley of the Great Salt Lake in Utah.

Before the United States formally declared war on Mexico, General Zachary Taylor defeats a superior Mexican force in the Battle of Palo Alto north of the Rio Grande River.

On May 13, 1846, the U.S. Congress overwhelmingly votes in favor of President James K. Polk’s request to declare war on Mexico in a dispute over Texas.

Anticipating the outbreak of war with Mexico, American settlers in California rebel against the Mexican government and proclaim the short-lived California Republic during the Bear Flag Revolt of 1846.

Representatives of Great Britain and the United States sign the Oregon Treaty, which settles a long-standing dispute with Britain over who controlled the Oregon territory. The treaty established the 49th parallel from the Rocky Mountains to the Strait of Georgia as the boundary between the United States and British Canada. The United States gained formal control over the future states of Oregon, Washington, Idaho and Montana; and the British retained Vancouver Island and navigation rights to part of the Columbia River.

An American naval captain occupies the small settlement of Yerba Buena, a site that will later be renamed San Francisco.

In an ominous sign of the troubles to come, the Donner Party finds a note warning the emigrants that their expected route through the mountains ahead is nearly impassable.

After a decade of debate about how best to spend a bequest left to America from an obscure English scientist, President James K. Polk signs the Smithsonian Institution Act into law on August 10, 1846.

Weeks behind schedule and the massive Sierra Nevada mountains still to be crossed, on what was likely September 10, 1846, the members of the ill-fated Donner Party realize they are running short of supplies and send two men ahead to California to bring back food.

Elizabeth Barrett elopes with Robert Browning on September 12, 1846.

German astronomer Johann Gottfried Galle discovers the planet Neptune at the Berlin Observatory.