<P> In 1798 , Spain revoked the treaty allowing American use of New Orleans , greatly upsetting Americans . In 1801 , Spanish Governor Don Juan Manuel de Salcedo took over from the Marquess of Casa Calvo , and restored the American right to deposit goods . However , in 1800 Spain had ceded the Louisiana territory back to France as part of Napoleon 's secret Third Treaty of San Ildefonso . The territory nominally remained under Spanish control , until a transfer of power to France on 30 November 1803 , just three weeks before the formal cession of the territory to the United States on 20 December 1803 . A further ceremony was held in St. Louis a few months later partially due to winter conditions impeding the arrival of news , Upper Louisiana , regarding the New Orleans formalities . The 9 -- 10 March 1804 event is remembered as Three Flags Day . </P>
<P> In 1798 , Spain revoked the treaty allowing American use of New Orleans , greatly upsetting Americans . In 1801 , Spanish Governor Don Juan Manuel de Salcedo took over from the Marquess of Casa Calvo , and restored the American right to deposit goods . However , in 1800 Spain had ceded the Louisiana territory back to France as part of Napoleon 's secret Third Treaty of San Ildefonso . The territory nominally remained under Spanish control , until a transfer of power to France on 30 November 1803 , just three weeks before the formal cession of the territory to the United States on 20 December 1803 . A further ceremony was held in St. Louis a few months later partially due to winter conditions impeding the arrival of news , Upper Louisiana , regarding the New Orleans formalities . The 9 -- 10 March 1804 event is remembered as Three Flags Day . </P>
when did louisiana become a part of the united states
when did louisiana become a part of the united states
20 December 1803
20 December 1803