Many French speakers left Illinois after it came under U.S. control following the Illinois Campaign of 1778 .
Illinois was still a bilingual Illinois for several more years before the flood of U.S.     settlers overwhelmed the old communities .
Many French speakers left Illinois after it came under U.S. control following the Illinois Campaign of 1778 , but Illinois was still a bilingual territory for several more years before the flood of Anglo - American settlers overwhelmed the old communities .