Whakaotihia te rerenga e whai ake nei: Mai i te tau 1805 ki muri, ka mahi tonu a Saint-Hilaire i roto i te "Grande Armée", e mau ana i nga whakahau wehewehe rereke. I te tau 1805, i whakahau ia i tetahi wehenga i roto i te IV Corps a Marshal Jean-de-Dieu Soult i te Pakanga o te Tuatoru o te Whakawhanaunga me te
I te Pakanga o Austerlitz, i arahi ia i tetahi whakaeke nui ki te papa o Pratzen, i whara kino ia i te timatanga o te whakaeke engari i mau tonu tana whakahau mo te toenga o te pakanga. I waenga i te 1806 me te 1807, i whawhai ia me te rereketanga i Jena, Eylau me Heilsberg. I whakawhiwhia ia ki a Count of the Empire i te tau 1808 ka riro i a ia he whakahau wehenga i roto i te ope taua o Tiamana, me tana pakanga ki te tonga o Tiamana me Austria. I te 22 o Paenga-whawha 1809, i wehewehe a Saint-Hilaire i a ia ano i raro i nga kanohi o Napoleon i te Pakanga o Eckmühl.