Lincoln believed that slavery would be rendered obsolete if its expansion into new territories were prevented, because these territories would be admitted to the Union as free states, and free states would come to outnumber slave states.
lˈɪŋkən bᵻlˈiːvd ðæt slˈeɪvɚɹi wʊd biː ɹˈɛndɚd ˈɑːbsəlˌiːt ɪf ɪts ɛkspˈænʃən ˌɪntʊ nˈuː tˈɛɹɪtˌoːɹiz wɜː pɹɪvˈɛntᵻd, bɪkˈʌz ðiːz tˈɛɹɪtˌoːɹiz wʊd biː ɐdmˈɪɾᵻd tə ðə jˈuːniən æz fɹˈiː stˈeɪts, ænd fɹˈiː stˈeɪts wʊd kˈʌm tʊ aʊtnˈʌmbɚ slˈeɪv stˈeɪts.