CIVIL WAR (1861–1865)
The American Civil War was a major conflict between the United States (the "Union") and the Southern slave states of the newly-formed Confederate States of America under Jefferson Davis. The Union included all of the free states and the five slaveholding border states and was led by Abraham Lincoln and the Republican Party. Republicans opposed the expansion of slavery into territories owned by the United States, and their victory in the presidential election of 1860 led seven Southern states to declare their secession from the Union even before Lincoln took office. The Union rejected secession, regarding it as rebellion.

Hostilities began on April 12, 1861, when Confederate forces attacked a U.S. military installation at Fort Sumter in South Carolina. Following this battle, four more Southern states declared their secession. In the war's first year, the Union assumed control of the border states and established a naval blockade as both sides massed armies and resources. In 1862, battles such as Shiloh and Antietam caused massive casualties unprecedented in U.S. military history. In September 1862, Lincoln's Emancipation Proclamation made ending slavery in the South a war goal, which complicated the South's manpower shortages.

In the East, Confederate commander Robert E. Lee won a series of victories over Union armies, but Lee's reverse at Gettysburg in early July, 1863 proved the turning point. The capture of Vicksburg and Port Hudson by Ulysses S. Grant completed Union control of the Mississippi River. Grant fought bloody battles of attrition with Lee in 1864, forcing Lee to defend the Confederate capital at Richmond, Virginia. Union general William Sherman captured Atlanta, Georgia, and began his famous March to the Sea, devastating a hundred-mile-wide swath of Georgia. Confederate resistance collapsed after Lee surrendered to Grant at Appomattox Court House in April 1865.

The war, the deadliest in American history, caused 620,000 soldier deaths and an undetermined number of civilian casualties, ended slavery in the United States, restored the Union by settling the issues of nullification and secession and strengthened the role of the Federal government.

WORLD WAR I (1914-1918)
World War I, also known as the First World War, the Great War and the War To End All Wars, was a global military conflict which took place primarily in Europe from 1914 to 1918. Over 40 million casualties resulted, including approximately 20 million military and civilian deaths. The conflict had a decisive impact on the history of the 20th century.

The Entente Powers, led by France, Russia, the United Kingdom and its colonies and dominions, and later Italy (from 1915) and the United States (from 1917), defeated the Central Powers, led by the Austro-Hungarian, German, and Ottoman Empires. Russia withdrew from the war after the revolution in 1917.

The fighting that took place along the Western Front occurred along a system of trenches, breastworks, and fortifications separated by an area known as no man's land. These fortifications stretched 475 miles (more than 600 kilometres) and defined the war for many. On the Eastern Front, the vast eastern plains and limited rail network prevented a trench warfare stalemate, though the scale of the conflict was just as large as on the Western Front. The Middle Eastern Front and the Italian Front also saw heavy fighting, while hostilities also occurred at sea, and for the first time, in the air.

World War I marked the end of the world order which had existed after the Napoleonic Wars, and was an important factor in the outbreak of World War II.

WORLD WAR II (1937-1945)
World War II, or the Second World War, was a worldwide military conflict; the amalgamation of two separate conflicts, one beginning in Asia, 1937, as the Second Sino-Japanese War and the other beginning in Europe, 1939, with the invasion of Poland. It is regarded as the historical successor to World War I.

This global conflict split a majority of the world's nations into two opposing military alliances: the Allies and the Axis. Spanning much of the globe, World War II resulted in the deaths of over 70 million people, making it the deadliest conflict in human history.

World War II was the most widespread war in history, and countries involved mobilized more than 100 million military personnel. Total war erased the distinction between civil and military resources and saw the complete activation of a nation's economic, industrial, and scientific capabilities for the purposes of the war effort; nearly two-thirds of those killed in the war were civilians. For example, nearly 11 million of the civilian casualties were victims of the Holocaust, which was largely conducted in Eastern Europe, and the Soviet Union.

The conflict ended in an Allied victory. As a result, the United States and Soviet Union emerged as the world's two leading superpowers, setting the stage for the Cold War for the next 45 years. Self determination gave rise to decolonization/independence movements in Asia and Africa, while Europe itself began traveling the road leading to integration.

KOREAN WAR (1950-1953)
The Korean War began as a civil war between two rival Korean regimes, each of which was supported by external powers. The North Korean Army moved south on June 25, 1950 to reunite the Korean peninsula, which had been formally divided since 1948. The conflict was then expanded by United States involvement as part of the larger Cold War between them and the Soviet Union. The main hostilities were during the period from June 25, 1950 until the armistice (ceasefire agreement) was signed on July 27, 1953.

The principal support for North Korea came from the People's Republic of China, with limited assistance from the Soviet Union in forms of combat advisors, military pilots, and weapons. South Korea was supported by the United Nations Command forces in Korea (U.N.) forces, consisting primarily of American troops, aircraft, artillery, naval support, weapons, and the threat of nuclear weapons. Before the conflict, North and South Korea existed as provisional governments competing for control over the Korean Peninsula after the division of Korea by the United States and the Soviet Union.

VIETNAM WAR (1959-1975)
The Vietnam War, also known as the Second Indochina War, the American War in Vietnam and the Vietnam Conflict, occurred from 1959 to April 30, 1975, concluding with the North Vietnamese military victory after more than 15 years. Over 1.4 million military personal were killed in the war (approximately 6% were members of the United States armed forces), while estimates of civilian fatalities range from 2 to 5.1 million. The war was fought between the Democratic Republic of Vietnam (North Vietnam) and the United States-supported Republic of Vietnam (South Vietnam). The war ended with the defeat of the Southern and American forces, and unification of Vietnam under the communist government of the North.

The U.S. and their allies deployed large numbers of troops to South Vietnam between the end of the First Indochina War in 1954 and 1973. U.S. military advisers first became involved in Vietnam in 1950, assisting French colonial forces. In 1956, these advisers assumed full responsibility for training the Army of the Republic of Vietnam. President John F. Kennedy increased America's troop numbers from 500 to 16,000, and President Lyndon Johnson dispatched a large number of troops beginning in 1965. Almost all U.S. military personnel departed after the Paris Peace Accords of 1973. The last American troops left the country on April 30, 1975.

GULF WAR (1990-1991)
The Gulf War or Persian Gulf War (2 August 1990 – 28 February 1991) was a conflict between Iraq and a coalition force from 35 nations authorized by the United Nations (UN) and led primarily by the United States in order to liberate Kuwait.

The conflict developed in the context of the Iran-Iraq War. The entry by Iraqi troops in Kuwait was met with immediate economic sanctions by some members of the UN Security Council against Iraq. The expulsion of Iraqi troops from Kuwait began in January 1991 and was a decisive victory for the coalition forces, which liberated Kuwait and penetrated Iraqi territory. Aerial and ground combat was confined to Iraq, Kuwait, and bordering areas of Saudi Arabia. Iraq also launched missiles against targets in Saudi Arabia and Israel in retaliation for their support of the invading forces in Kuwait.

IRAQ WAR (2003-present)
The Iraq War, also known as the Occupation of Iraq, the Second Gulf War, or Operation Iraqi Freedom, is an ongoing conflict which began on March 20, 2003 with the United States-led invasion of Iraq.

The main rationale for the Iraq War offered by U.S. President George W. Bush, former Prime Minister of the United Kingdom Tony Blair, former Prime Minister of Spain José María Aznar and their domestic and foreign supporters, was the belief that Iraq possessed and was actively developing weapons of mass destruction (WMD). Diplomats from countries on the U.N. Security Council that opposed the war made statements that supported this belief. These weapons, it was argued, posed a threat to the United States, its allies and interests. In the 2003 State of the Union Address, Bush claimed that the U.S. could not wait until the threat from Iraqi leader Saddam Hussein became imminent. After the invasion, however, no evidence was found of the WMD or programs the administration claimed existed. Some U.S. officials cited claims of a connection between Saddam Hussein and al-Qaeda. No evidence of any operational or collaborative relationship with al-Qaeda has been found.

The war began on March 20, 2003, when a largely American force supported by small contingents from Great Britain, Australia, Denmark and Poland invaded Iraq. The invasion soon led to the defeat and flight of Saddam Hussein. The U.S.-led coalition occupied Iraq and attempted to establish a new democratic government; however it failed to restore order in Iraq. The unrest led to asymmetric warfare with the Iraqi insurgency, civil war between many Sunni and Shia Iraqis and al-Qaeda operations in Iraq. Coalition nations have begun to withdraw troops from Iraq as public opinion favoring troop withdrawal increases and as Iraqi forces begin to take responsibility for security. The causes and consequences of the war remain controversial.