Editing 1803 (section)

=== July–September ===
* [[July 4]] – The [[Louisiana Purchase]] is announced to the American people.
* [[July 5]] – The [[convention of Artlenburg]] leads to the French occupation of [[Hanover]] (which had been ruled by the British king).
* [[July 23]] – [[Robert Emmet]]'s uprising in Ireland begins.
* [[July 26]] – The [[wagonway]] between [[Wandsworth]] and [[Croydon]] is opened, being the first public railway line in England.
* [[August 3]] – The British begin the [[Second Anglo-Maratha War]], against the [[Scindia Dynasty]] of [[Gwalior]].
* [[August 31]] – The [[Lewis and Clark Expedition]] begins as Lewis and a crew of 11 depart on the Ohio River. 
* [[September 6]] – [[John Dalton]], British scientist, begins using symbols to represent the atoms of different elements.
* [[September 11]] – [[Second Anglo-Maratha War]]: [[Battle of Delhi (1803)|Battle of Delhi]] – British troops under [[Gerard Lake, 1st Viscount Lake|General Lake]] defeat the [[Maratha Confederacy|Marathas]] of [[Scindia Dynasty|Scindia]]'s army, under General Louis Bourquin.
* [[September 20]] – Irish rebel [[Robert Emmet]] is executed.
* [[September 23]] – [[Second Anglo-Maratha War]]: [[Battle of Assaye]] – British-led troops defeat [[Maratha]] forces.