February 23 - President-elect Abraham Lincoln arrives secretly in Washington, D.C., after an assassination attempt in Baltimore, Maryland.
February 27 - A crowd in Warsaw protesting against Russian rule over Poland is fired upon by Russian troops, killing five protesters.
February 28 - Colorado is organized as a United States territory.
March 2 - Nevada Territory and Dakota Territory are organized as political divisions of the United States.
March 2 - Emancipation reform of 1861 in Russia: Tsar Alexander II signed the emancipation reform into law, abolishing Russian serfdom.
March 3 - Alexander II of Russia signs the Emancipation Manifesto, freeing serfs.
March 4 - First national flag of the Confederate States of America (the 'Stars and Bars') adopted.
March 4 - Abraham Lincoln inaugurated as the 16th president of the United States.
March 5 - The "Stars and Bars" is adopted as the flag of the Confederate States of America.
March 8 - St. Augustine, Florida, surrenders to Union forces.
March 10 - El Hadj Umar Tall seizes the city of Segou, destroying the Bambara Empire of Mali.
March 11 - American Civil War: The Constitution of the Confederate States of America is adopted.
March 16 - Edward Clark became Governor of Texas, replacing Sam Houston, who was evicted from the office for refusing to take an oath of loyalty to the Confederacy.
March 17 - The Kingdom of Italy (1861-1946) is proclaimed.
March 19 - The First Taranaki War ends in New Zealand.
March 20 - An earthquake completely destroys Mendoza, Argentina.
April 12 - American Civil War: The war begins with Confederate forces firing on Fort Sumter, in the harbor of Charleston, South Carolina.
April 13 - American Civil War: Fort Sumter surrenders to Confederate forces.
April 17 - American Civil War: Virginia secedes from the United States.
April 19 - American Civil War: Baltimore riot of 1861, a pro-Secession mob in Baltimore, Maryland, attacks United States Army troops marching through the city.
April 20 - American Civil War: Robert E. Lee resigns his commission in the United States Army in order to command the forces of the state of Virginia.
April 25 - American Civil War: The Union Army arrives in Washington, D.C.
April 27 - President of the United States Abraham Lincoln suspends the writ of habeas corpus.
April 29 - American Civil War: Maryland's House of Delegates votes not to secede from the Union.
May 6 - American Civil War: Arkansas secedes from the Union.
May 8 - American Civil War: Richmond, Virginia is named the capital of the Confederate States of America.
May 13 - American Civil War: Queen Victoria of Britain issues a "proclamation of neutrality" which recognizes the breakaway states as having belligerent rights.
May 13 - The Great Comet of 1861 is discovered by John Tebbutt of Windsor, New South Wales, Australia.
May 14 - The Canellas meteorite, an 859-gram chondrite-type meteorite, strikes the earth near Barcelona, Spain.
May 20 - American Civil War: The state of Kentucky proclaims its neutrality, which will last until September 3 when Confederate forces enter the state.
May 24 - American Civil War: Union troops occupy Alexandria, Virginia.
June 3 - Battle of Philippi (also called the Philippi Races)
June 8 - American Civil War: Tennessee secedes from the Union.
July 21 - American Civil War: First Battle of Bull Run
July 25 - American Civil War: the Crittenden-Johnson Resolution is passed by the U.S. Congress stating that the war is being fought to preserve the Union and not to end slavery.
July 26 - American Civil War: George B. McClellan assumes command of the Army of the Potomac following a disastrous Union defeat at the First Battle of Bull Run.
August 5 - American Civil War: In order to help pay for the war effort, the United States government levies the first income tax as part of the Revenue Act of 1861 (3% of all incomes over US $800; rescinded in 1872).
August 5 - The United States Army abolishes flogging.
August 10 - American Civil War: Battle of Wilson's Creek
August 27 - Union (Northern) forces attack Cape Hatteras, North Carolina.
August 29 - American Civil War: US Navy squadron captures forts at Hatteras Inlet, North Carolina.
September 3 - American Civil War: Confederate General Leonidas Polk invades neutral Kentucky, prompting the state legislature to ask for Union assistance.
September 6 - American Civil War: Forces under Union General Ulysses S. Grant bloodlessly capture Paducah, Kentucky, which gives the Union control of the mouth of the Tennessee River.
October 9 - American Civil War: Battle of Santa Rosa Island
October 21 - American Civil War: Battle of Ball's Bluff
October 23 - U.S. President Abraham Lincoln suspends the writ of habeas corpus in Washington, D.C., for all military-related cases.
October 24 - The First Transcontinental Telegraph line across the United States is completed, spelling the end for the 18-month-old Pony Express.
October 25 - The Toronto Stock Exchange was created.
October 26 - The Pony Express officially ceased operations.
November 2 - American Civil War: Western Department Union General John C. Fremont is relieved of command and replaced by David Hunter.
November 4 - The University of Washington opens in Seattle, Washington as the Territorial University
November 6 - American Civil War: Jefferson Davis is elected president of the Confederate States of America.
November 7 - American Civil War: Battle of Belmont: In Belmont, Missouri, Union forces led by General Ulysses S. Grant overrun a Confederate camp but are forced to retreat when Confederate reinforcements arrive.
November 8 - American Civil War: The "Trent Affair"
November 9 - The first documented football match in Canada is played at University College, University of Toronto.
November 20 - Secession ordinance is filed by Kentucky's Confederate government.
November 21 - American Civil War: Confederate President Jefferson Davis appoints Judah Benjamin secretary of war.
December 9 - American Civil War: The Joint Committee on the Conduct of the War is established by the U.S. Congress.
December 10 - American Civil War: the Confederate States of America accept a rival state government's pronouncement that declares Kentucky to be the 13th state of the Confederacy.
December 21 - Medal of Honor: Public Resolution 82, containing a provision for a Navy Medal of Valor, is signed into law by President Abraham Lincoln.
December 26 - American Civil War: The Trent Affair: Confederate diplomatic envoys James M. Mason and John Slidell are freed by the United States government, thus heading off a possible war between the United States and Britain.