After Holland was annexed to the First French Empire, Collaert entered French service as a general of brigade and was assigned to garrison duty for two years. In 1813 he was appointed to command a French cavalry brigade in the German Campaign. That year he led his horsemen at Dresden, Leipzig and Sainte-Croix-en-Plaine. In 1814 he fought at La Rothière, Mormant and Saint-Dizier. Going to the newly created Kingdom of the Netherlands in 1815, he was soon appointed lieutenant general of its cavalry. He was badly wounded at Waterloo on 18 June and died of his injuries a year later.