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- April 30 - Ambassadors James Monroe and Robert R. Livingston of the United States leave Paris for London when negotiations with French officials over the purchase of the city of New Orleans and its environs break down.
- May 4 - The London Agreement of 1803 is drafted between the two American ambassadors and British officials representing King George III and Prime Minister Adington. It states that the United States and Great Britain shall divide Louisiana along the 42nd parallel north with the southern section going to the United States and the norther section going to the British Empire.
- October 29 - The London Agreement is relunctantly approved by the United States Senate with a slim majority vote of 18 - 16.