In a strongly acidic medium, the basic amino group ($-NH_2$) of aniline readily accepts a proton to form an anilinium ion ($-NH_3^+$). While the free amino group is electron-donating and ortho/para-directing, the anilinium ion carries a positive charge on nitrogen, making it strongly electron-withdrawing via the $-I$ effect. This deactivates the benzene ring and directs the incoming electrophile (nitro group) to the meta-position. Consequently, nitration of aniline in a strongly acidic medium yields a significant proportion of m-nitroaniline.