In the Austro-Prussian War of 1866, Austria contested with Prussia the position of leadership among the German states. The Kingdom of Italy seized the opportunity to capture Venetia from Austrian rule and allied itself with Prussia.
German Civil War was fought in 1866 between the Austrian Empire and the Kingdom of Prussia, with each also being aided by various allies within the German Confederation. Prussia had also allied with the Kingdom of Italy, linking this conflict to the Third Independence War of Italian unification. The Austro-Prussian War was part of the wider rivalry between Austria and Prussia, and resulted in Prussian dominance over the German states.
In February 1866, Johnson vetoed legislation extending the Freedmen's Bureau and expanding its powers; Congress was unable to override the veto. Afterward, Johnson denounced Radical Republicans Representative Thaddeus Stevens and Senator Charles Sumner, along with abolitionist Wendell Phillips, as traitors.
A year after the war's end, in April 1866, four women of Columbus gathered together to decorate the graves of the Confederate soldiers. They also felt moved to honor the Union soldiers buried there and to note the grief of their families, by decorating their graves as well. The story of their gesture of humanity and reconciliation is held by some writers as the inspiration of the original Memorial Day despite it occurring last among the claimed inspirations.
In April 1866, the Prussian representative in Florence signed a secret agreement with the Italian government, committing each state to assist the other in a war against Austria. The next day, the Prussian delegate to the Frankfurt assembly presented a plan calling for a national constitution, a directly elected national Diet, and universal suffrage.
Formerly enslaved people in Galveston celebrated after the announcement. Freedmen in Texas organized the first of what became the annual celebration of "Jubilee Day" on June 19. Early celebrations were used as political rallies to give voting instructions to newly freed slaves. Early independence celebrations often occurred on January 1 or 4.
Austria tried to persuade the Italian government to accept Venetia in exchange for non-intervention. However, on 8 April, Italy and Prussia signed an agreement that supported Italy's acquisition of Venetia, and on 20 June Italy issued a declaration of war on Austria.
By 1866, nearly 7,000 patients were being treated per year at the New York Infirmary, and Blackwell was needed back in the United States. The parallel project fell through, but in 1868, a medical college for women adjunct to the infirmary was established. It incorporated Blackwell's innovative ideas about medical education – a four-year training period with much more extensive clinical training than previously required.
The Orissa famine of 1866 affected the east coast of India from Madras northwards, an area covering 180,000 miles and containing a population of 47,500,000; the impact of the famine, however, was greatest in Orissa, now Odisha, which at that time was quite isolated from the rest of India. In Odisha, one third of the population died due to famine. In Odisha alone, at least 1 million people, a third of the population, died in 1866, and overall in the region approximately 4 to 5 million died in the two-year period
In the late-summer of 1866, the president embarked on a national "Swing Around the Circle" speaking tour, where he asked his audiences for their support in his battle against the Congress and urged voters to elect representatives to Congress in the upcoming midterm election who supported his policies.