The Polish-Italian Legion fought its first engagement at the Siege of Kłodzko and then was transferred to Kingdom of Westphalia in October 1807 and was placed in garrison in the capital, Kassel where it was recruited to full authorized strength from Poles in French occupied territory.

The infantry was reorganized to the 1808 pattern of six company battalions in April of that year in conformance with the new French organization decreed on 18 February 1808.

All of the personnel of the Legion were to be of Polish ethnicity except for the company clerks, the fourriers, battalion adjutant non-commissioned officers and paymasters, who were to be French.

On 24 March 1809 at the Battle of Yevenes, 600 of Polish lancers of the Legion of the Vistula lost all their squadron banners (which had not been defended because nobody knew they were hidden in the carriage) when they had cut through the ambush set by 5,000 of Spaniards.

Napoleon ordered a Second Vistula Legion formed from prisoners taken after his defeat of the Austrians at the Battle of Wagram in July 1809.

The Vistula Regiment was again reformed early in 1814 at its depot at Sedan with all the remaining Polish infantrymen in French service being transferred to it.