March 4, 1845 - James K. Polk inaugurated at 11th president.

July 4, 1845 - American writer Henry David Thoreau began a two-year experiment in simple living at Walden Pond near Concord, MA.

October 13, 1845 - A majority of the citizens of the independent Republic of Texas approve a proposed constitution, that when accepted by the Congress, will make Texas the 28th American state.

November 4, 1845 - Americans observed the first national election day in accordance with Congressional legislation passed earlier in the year.

December 2, 1845 - Making his first annual address to Congress, President James K. Polk belligerently reasserts the 1823 Monroe Doctrine and calls for aggressive American expansion into the West. Polk's aggressive expansionist program created the outline of the modern American nation; reflected his willingness to use force to create a nation stretching across the continent, felt that such expansion was part of America's "manifest destiny." Six months after his speech to Congress, Polk's decision to annex the Republic of Texas led to war with Mexico. 1848 - Americans emerge victorious; Treaty of Guadalupe Hidalgo gave Polk precisely what he wanted: the vast northern provinces of the Mexican empire that would one day become the states of Texas, California, Arizona, New Mexico, Nevada, and Utah.

December 10, 1845 - President James Polk make a bold move to radically expand the burgeoning United States; gave Congressman John Slidell the go-ahead to settle a border dispute concerning Texas, as well as to purchase New Mexico and California, from Mexico; Sidell paid $5 million for New Mexico and $25 million for California; however, Mexico refused the offer, emboldening the president to marshal a war effort in the name of "re-annexing" the territory.

December 29, 1845 - Before Polk could take office, President John Tyler beat him to the punch by securing a congressional resolution calling for annexation. With the strong approval of most Texans, Polk signed the legislation making Texas an American state. December 29, 1845 - U.S. Customs conferred fiscal legitimacy on Texas's admission to the Union and began to collect revenue in the state.

January 1846 - Prime Minister Sir Robert Peel and his Conservative government passed new Corn Laws in Britain (first passed in 1815); reduced the duty on oats, barley and wheat to the insignificant sum of one shilling per quarter.

January 5, 1846 - U.S. House of Representatives reversed its long-standing policy of "free and open" occupation in the disputed Oregon Territory, passed a resolution calling for an end to British-American sharing of the region; expansionistic President James Polk coveted Oregon Territory up to the 49th parallel (the modern-day border with Canada); British agreed to abandon their claim to the area north of the Columbia and accept the 49th parallel as a border (Hudson Bay Company already had decided to relocate its principal trading post from the Columbia River area to Vancouver Island); left Polk free to pursue war with Mexico for control of the Southwest.

May 13, 1846 - The United States declared that a state of war existed against Mexico.

June 14, 1846 - A group of U.S. settlers in Sonoma, CA rebel against the Mexican government and proclaim the short-lived California Republic (Bear Flag revolt). A party of 33 Americans, under the leadership of Ezekiel Merritt and William Ide, invaded the largely defenseless Mexican outpost of Sonoma, just north of San Francisco. Merritt and his men surrounded the home of the retired Mexican general, Mariano Vallejo, and informed him that he was a prisoner of war. Vallejo, who was actually a strong supporter of American annexation, was more puzzled than alarmed by the rebels. He invited Merritt and a few of the other men into his home to discuss the situation over brandy. After several hours passed, Ide went in and spoiled what had turned into pleasant chat by arresting Vallejo and his family. Having won a bloodless victory at Sonoma, Merritt and Ide then proceeded to declare California an independent republic. With a cotton sheet and some red paint, they constructed a makeshift flag with a crude drawing of a grizzly bear, a lone red star (a reference to the earlier Lone Star Republic of Texas), and the words "California Republic" at the bottom. From then on, the independence movement was known as the Bear Flag Revolt. July 1, 1846 - American army officer and explorer John C. Fremont officially took command of the "Bear Flaggers" and occupied the unguarded presidio of San Francisco. July 7, 1846 - Fremont learned that American forces under Commodore John D. Sloat had taken Monterey without a fight and officially raised the American flag over California. Since the ultimate goal of the Bear Flaggers was to make California part of the U.S., they now saw little reason to preserve their "government." Three weeks after it had been proclaimed, the California Republic quietly faded away. 1850 - Bear Flag became the official state flag when California joined the union.

June 15, 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. 1818 - a U.S.-British agreement had established the border along the 49th parallel from Lake of the Woods in the east to the Rocky Mountains in the west. The two nations also agreed to a joint occupation of Oregon territory for 10 years, an arrangement that was extended for an additional 10 years in 1827. After 1838 - the issue of who possessed Oregon became increasingly controversial, especially when mass American migration along the Oregon Trail began in the early 1840s. 1844 - Democrat James K. Polk successfully ran for president under the platform "Fifty-four forty or fight," which referred to his hope of bringing a sizable portion of present-day Vancouver and Alberta into the United States. However, neither President Polk nor the British government wanted a third Anglo-American war, and on June 15, 1846, the Oregon Treaty, a compromise, was signed. By the terms of the agreement, the U.S. and Canadian border was extended west along the 49th parallel to the Strait of Georgia, just short of the Pacific Ocean.

July 7, 1846 - U.S. annexation of California was proclaimed at Monterey after the surrender of a Mexican garrison.

August 8, 1846 - President James Polk requests 2 million dollars to purchase land from Mexico following the Mexican-American War. David Wilmot of Pennsylvania attaches the "Wilmot Proviso" (based on the Northwest Ordinance of 1787) to this bill. It passes in the House but is tabled in the Senate. Stated: "the acquisition of any territory from the Republic of Mexico by the United States...neither slavery nor involuntary servitude shall ever exist in any part of said territory, except for crime, whereof the party shall first be duly convicted." February 8, 1847 - Wilmot offered Proviso to a Mexican War appropriations bill proposed by House member Preston King (D-NY).

August 18, 1846 - U.S. forces led by Gen. Stephen W. Kearny captured Santa Fe, N.M.

August 22, 1846 - The United States annexed New Mexico.

December 28, 1846 - Iowa became the 29th state to be admitted to the Union.

January 16, 1847 - General Robert Stockton appointed explorer and mapmaker John C. Fremont as territorial governor of the newly won American territory. Fremont had already won national acclaim for his leadership of two important explorations of the West with the military's Corps of Topographical Engineers; published meticulously accurate maps of the Far West, they became indispensable guides for the growing numbers of overland emigrants heading for California and Oregon. A dispute broke out within the army over the legitimacy of Fremont's appointment, and the young captain's detractors accused him of mutiny, disobedience, and conduct prejudicial to military discipline. Recalled to Washington for a court martial, Fremont was found guilty of all three charges, and his appointment to take the position of governor was revoked. Though President Polk pardoned him and ordered him back to active duty in the army, Fremont was deeply embittered, and he resigned from the military and returned to California a private citizen. Although he never regained the governorship of California, the turmoil of Fremont's early political career did not harm his future prospects. In 1851, citizens of California elected him a senator, and became the territorial governor of Arizona in 1878.

March 9, 1847 - U.S. forces under General Winfield Scott invade Mexico three miles south of Vera Cruz. Encountering little resistance from the Mexicans massed in the fortified city of Vera Cruz, by nightfall the last of Scott's 10,000 men came ashore without the loss of a single life. It was the largest amphibious landing in U.S. history and not surpassed until World War II; March 29, 1847 - with very few casualties, the Americans had taken the fortified city and its massive fortress, San Juan de Ulua. April 1847 - Scott began his devastating march to Mexico City, which ended on September 14, when U.S. forces entered the Mexican capital and raised the American flag over the Hall of Montezuma.

March 29, 1847 - U.S. forces led by Gen. Winfield Scott occupied the city of Veracruz after its Mexican defenders capitulated.

July 1, 1847 - The United States Post Office issued its first stamps, a five-cent stamp honoring Benjamin Franklin and a ten-cent stamp for George Washington.

July 24, 1847 - Mormon leader Brigham Young and 148 of his followers arrived in the valley of the Great Salt Lake in present-day Utah. Began building the future Salt Lake City at the foot of the Wasatch Mountains. 1850 - President Millard Fillmore named Brigham Young the first governor of the U.S. territory of Utah, and the territory enjoyed relative autonomy for several years. 1852 - 16,000 Mormons had come to the valley, some in wagons and some dragging handcarts. 1857 - President James Buchanan removed Young, who had 20 wives, from his position as governor and sent U.S. Army troops to Utah to establish federal authority. 1869 - 80,000 Mormons had made the trek to their promised land. 1877 -  succeeded by John Taylor as president of the church. 1890 - Wilford Woodruff, the new president of the Mormon church, issued his Manifesto, renouncing the traditional practice of polygamy and reducing the domination of the church over Utah communities. 1896 - territory of Utah entered the Union as the 45th state.

July 26, 1847 - The Republic of Liberia, formerly a colony of the American Colonization Society, declares its independence. Under pressure from Britain, the United States hesitantly accepted Liberian sovereignty, making the West African nation the first democratic republic in African history. A constitution modeled after the U.S. Constitution was approved. 1848 - Joseph Jenkins Roberts was elected Liberia's first president. 1816 - American Colonization Society founded by American Robert Finley to return freed African American slaves to Africa. 1821 - the American Colonization Society founded the colony of Liberia south of Sierra Leone as a homeland for former slaves outside British jurisdiction.

August 26, 1847 - Liberia was proclaimed an independent republic.

February 2, 1848 - The Treaty of Guadalupe Hidalgo was signed in Guadalupe Hidalgo, a city north of the capital where the Mexican government had fled as US troops advanced; ended the Mexican War; added an additional 525,000 square miles to United States territory, including the area that would become the states of Texas, California, Nevada, Utah, New Mexico and Arizona, as well as parts of Colorado and Wyoming (US paid Mexico $15 million); Polk ignited the Mexican-American War when he sent his Commanding General of the Army Zachary Taylor, a Republican, and his troops to claim territory along the Rio Grande River between the U.S. and Mexico. June 19, 1848 - Treaty proclaimed by the President.

February 2, 1848 - Former President John Quincy Adams suffered a stroke on the floor of the House of Representatives in Washington, DC. He died two days later.

February 21, 1848 - Karl Marx, with the assistance of Friedrich Engels, published The Communist Manifesto in London by a group of German-born revolutionary socialists known as the Communist League; proclaimed that "the history of all hitherto existing society is the history of class struggles" and that the inevitable victory of the proletariat, or working class, would put an end to class society forever. February 22, 1848 - revolution broke out in France over the banning of political meetings held by socialists and other opposition groups. Isolated riots led to popular revolt; February 24, 1848 - King Louis-Philippe was forced to abdicate. The revolution spread like brushfire across continental Europe. Marx was in Paris on the invitation of the provincial government when the Belgian government, fearful that the revolutionary tide would soon engulf Belgium, banished him. Later that year, he went to the Rhineland, where he agitated for armed revolt.

February 26, 1848 - The Second French Republic was proclaimed.

March 10, 1848 - The Senate ratified the Treaty of Guadalupe Hidalgo, ending the war with Mexico.

May 29, 1848 - Wisconsin became the 30th state of the union. 1832 - Black Hawk War ended Native American resistance to white settlement. 1836 - after several decades of governance as part of other territories, Wisconsin was made a separate entity, with Madison, located midway between Milwaukee and the western centers of population, marked as the territorial capital. 1840 - population in Wisconsin had risen above 130,000, but the people voted against statehood four times, fearing the higher taxes that would come with a stronger central government. 1848 - Wisconsin citizens, envious of the prosperity that federal programs brought to neighboring Midwestern states, voted to approve statehood. Wisconsin entered the Union the next May.

August 9, 1848 - The Free-Soil Party nominated Martin Van Buren for president at its convention in Buffalo, NY.

August 14, 1848 - The Oregon Territory was established.

August 19, 1948 - News of gold discovered in California in January of 1848 made it to the East Coast; New York Herald published news of discovery.

May 30, 1848 - Mexico ratifies treaty giving U.S.; New Mexico, California and parts of Nevada, Utah, Arizona and Colorado in return for $15 million.

July 19, 1848 - A pioneer women's rights convention, first ever held in the United States, convened at the Wesleyan Chapel in Seneca Falls, NY with almost 200 women in attendance. The convention was organized by Lucretia Mott and Elizabeth Cady Stanton, two abolitionists who met at the 1840 World Anti-Slavery Convention in London. As women, Mott and Stanton were barred from the convention floor, and the common indignation that this aroused in both of them was the impetus for their founding of the women's rights movement in the United States. Stanton read the "Declaration of Sentiments and Grievances," a treatise that she had drafted over the previous few days. Stanton's declaration was modeled closely on the Declaration of Independence, and its preamble featured the proclamation, "We hold these truths to be self-evident: that all men and women are created equal; that they are endowed by their Creator with certain inalienable rights..." The Declaration of Sentiments and Grievances then detailed the injustices inflicted upon women in the United States and called upon U.S. women to organize and petition for their rights. 1920 - 19th Amendment was adopted, granted American women the constitutionally protected right to vote.

November 7, 1848 - Zachary Taylor is elected president in the first US presidential election held in every state on the same day.

December 5, 1848 - President James K. Polk triggered the Gold Rush of '49 by confirming that gold had been discovered in California.

February 14, 1849 - The first photograph of a US President (James Polk) was taken by Matthew Brady, in New York City.

March 3, 1849 - US Department of the Interior, initially called the Home Department, was established.

March 3, 1849 - Congress created the Minnesota Territory.