[1] At the age of 15, de Lassus joined the Spanish army and rose to the rank of lieutenant colonel and earned a place in the Walloon Guards.

[3] During the French Revolution, de Lassus's parents fled to Spanish Louisiana in 1791, settling in New Bourbon, an area near modern Ste.

[5] His first act upon taking command in St. Louis was to order a population census of Upper Louisiana, which recorded 6,028 people—4,948 whites, 883 enslaved people, and 197 freedmen.

De Lassus began making the preparations and in March 1804 he oversaw the Three Flags Day ceremonies as power was transferred from Spain to France to the United States.

He remained in St. Louis until October 1804, managing lingering details of the transfer of the territory until he received orders to convey his soldiers and munitions to New Orleans.

[8] His next posting was to succeeded Carlos de Grand Pré as Spanish governor of the Baton Rouge District in December 1808.

Planters in Feliciana complained that Spanish officials were corrupt and nonresponsive, and de Lassus felt the small garrison at Baton Rouge was woefully inadequate for maintaining order in the territory.

[10] Feleciana planters, displeased with de Lassus's response to their concerns and accusing him of corruption, began organizing popular conventions in the summer of 1810.

On the morning of September 23, 1810, an armed group led by Philemon Thomas successfully captured Fort San Carlos in Baton Rouge, imprisoned de Lassus, and declared an independent Republic of West Florida.