Anyone who tries to change the system will have to 'reckon with' me. = Anyone who tries to change the system will have me to 'reckon with'. [=I will oppose/fight anyone who tries to change the system]
She 'was reckoned' [='considered'] to be among the group's leaders. = She 'was reckoned' as one of the group's leaders.
When he won his first three matches, the other players realized that he was 'a force/man to be reckoned with'.
(..) the return of spring fashion. Fresh and breezy... unlike the pompous styles of wartime. l'll make mincemeat of you! ''You'll reckon with me!'' Another sad page... in the tragedy of our men in Russia. The Defense Ministry... has released a new list (..)1988 Cinema Paradiso
(..) They smoke like chimneys. No. Go buy a ticket. No! What was that? l'll make mincemeat of you! You'll reckon with me! Signora Anna! Alfredo. Don't blame me. Your wife told me to bring your lunch. l told my mother (..)1988 Cinema Paradiso