Rhode Island abolished slavery in 1774. It was followed by Vermont (1777), Pennsylvania (1780), Massachusetts (1781), New Hampshire (1783), Connecticut (1784), New York (1799) and New Jersey (1804). The new states of Maine, Michigan, Wisconsin, Ohio, Indiana, Kansas, Oregon, California and Illinois also did not have slaves. The importation of slaves from other countries was banned in 1808. However, the selling of slaves within the southern states continued.

Conflict grew between the northern and southern states over the issue of slavery. The northern states were going through an industrial revolution and desperately needed more people to work in its factories. Industrialists in the North believed that, if freed, the slaves would leave the South and provide the labour they needed. The North also wanted tariffs on imported foreign goods to protect their new industries. The South was still mainly agricultural and purchased a lot of goods from abroad and was therefore against import tariffs.

The vast majority of European immigrants that arrived at the beginning of the 19th century opposed slavery. Leaders of immigrant organizations such as Carl Schurz (Germany),Tufve Nilsson Hasselquist (Sweden) and Hans Christian Heg (Norway) became involved in the struggle for abolition.

Abraham Lincoln, a northern opponent of slavery, was elected as president in 1861. It has been pointed out that without the support of an overwhelming number of immigrants, Lincoln would have lost the election. After Lincoln became president eleven southern states (Alabama, Arkansas, Florida, Georgia, Louisiana, North Carolina, South Carolina, Tennessee, Texas and Virginia) decided to leave the Union and form their own separate government in the South.

This resulted in the outbreak of the American Civil War. European immigrants joined the Union Army in large numbers. Over 6,000 Germans in New York immediately responded to Lincoln's call for volunteers. Another 4,000 Germans in Pennsylvania also joined. The French community were keen to show its support of the Union. The Lafayette Guards, an entirely French company, was led by Colonel Regis de Trobriand. The 55th New York Volunteers was also mainly composed of Frenchmen.

It is estimated that over 400,000 immigrants served with the Union Army. This included 216,000 Germans and 170,000 Irish soldiers. There were several important German born military leaders such as August Willich, Carl Schurz, Alexander Schimmelfennig, Peter Osterhaus, Franz Sigel and Max Weber. One Irish immigrant, Thomas Meagher, became a highly successful commander in the war. Another important military figure was the Norwegian soldier, Hans Christian Heg, who was mainly responsible for establishing the Fifteenth Wisconsin Volunteers (also known as the Scandinavian Regiment).

An estimated 4,000 Swedes fought in the Union Army. Hans Mattson had a successful career as a colonel in the Union Army and later became Secretary of State for Minnesota (1870-1872).

At Chickamauga 63% of the Scandinavian Regiment were killed, wounded or captured. This included Colonel Hans Christian Heg, the highest ranking officer in Wisconsin to die in the war. Heavy losses were also experienced by the Scandinavian Regiment at Pickett's Mill (27th May, 1864).

The Confederate Army had few foreign-born soldiers. There main support came from Irish immigrants and an estimated 40,000 joined the forces fighting the Union Army. The Irish tended to support the Democratic Party rather than the Republican Party. This led to the Irish taking part in draft riots in Boston and New York City during the summer of 1863.

The Irish had little sympathy for slaves as they feared that if they were given their freedom they would move north and threaten the jobs being done by Irish immigrants. One leading Irish-American politician, John Mitchel, wrote in his newspaper, The Citizen in 1856: "He would be a bad Irishman who voted for principles which jeopardized the present freedom of a nation of white men, for the vague forlorn hope of elevating blacks to a level for which it is at least problematical whether God and nature ever intended them."