Editing 1866 (section)

=== July–September ===
* [[July 3]] – [[Battle of Königgrätz]]: the [[Prussia]]n army under [[Wilhelm I of Germany|King Wilhelm]] and [[Helmuth von Moltke the Elder|Helmuth von Moltke]] defeats the Austrian army of [[Ludwig von Benedek]], leading to a decisive Prussian victory in the Austro-Prussian War.
* [[July 5]] – [[Princess Helena of the United Kingdom|Princess Helena]], third daughter of [[Queen Victoria]], marries [[Prince Christian of Schleswig-Holstein]].
* [[July 10]] – [[Reconstruction Treaties|Reconstruction Treaty]] with [[Choctaw]] & [[Chickasaw]], [[Timeline of abolition of slavery and serfdom|completing the abolition of slavery]] in the United States; see also [[Choctaw freedmen]].
* [[July 13]] (July 1 [[Old Style]]) – The first Constitution of [[Romania]] is issued.
* [[July 20]] – Austro-Prussian War: Naval [[Battle of Lissa (1866)|Battle of Lissa]] – The Austrian fleet under [[Wilhelm von Tegetthoff]] defeats the Italian fleet of [[Carlo di Persano]].
* [[July 22]] – Austro-Prussian War: [[Battle of Blumenau]] – Austrians defend Bratislava against the Prussian army, concluding the fighting in the war.
* [[July 24]] – [[Reconstruction Era|Reconstruction]]: [[Tennessee]] becomes the first [[U.S. state]] to be readmitted to the [[United States|Union]] following the [[American Civil War]].
* [[July 25]] – The [[United States Congress]] passes legislation authorizing the four-star rank of [[General of the Army (United States)#Post–American Civil War era|General of the Army]] (later reestablished as a five-star rank); Lieutenant General [[Ulysses S. Grant]] becomes the first to have this rank.
[[File:Atlantic cable Map.jpg|thumb|[[July 27]]: [[Atlantic Cable]] completed]]
* [[July 27]] – The {{SS|Great Eastern}} successfully completes laying the [[transatlantic telegraph cable]] between [[Valentia Island]], Ireland, and [[Heart's Content, Newfoundland and Labrador|Heart's Content, Newfoundland]], permanently restoring a communications link.
* [[July 28]] – The ''[[Metric Act of 1866]]'' becomes law and legalizes the standardization of [[weights and measures]] in the United States.
* [[August 23]] – The [[Peace of Prague (1866)|Treaty of Prague]] formally ends the [[Austro-Prussian War]]. The [[Limburg (Netherlands)|Duchy of Limburg]] leaves the [[German Confederation]].
* September – The [[Great Tea Race of 1866]] ends in London, narrowly won by the [[clipper ship]] ''[[Taeping]]''.
* [[September 22]] – [[Paraguay]] successfully [[Battle of Curupayty|defends Curupayty]] against the Triple Alliance in the [[Paraguayan War]], killing more than 5,000 while sustaining just about 50 casualties.